DATAMONGO-2609 - Release version 3.0.4 (Neumann SR4).

Now toDocument calls toDocument on items correctly.

Original Pull Request: #883

.travis.yml

@@ -1,40 +0,0 @@
-language: java
-
-before_install:
-  - mkdir -p downloads
-  - mkdir -p var/db var/log
-  - if [[ ! -d downloads/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION} ]] ; then cd downloads && wget https://fastdl.mongodb.org/linux/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}.tgz && tar xzf mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}.tgz && cd ..; fi
-  - downloads/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}/bin/mongod --version
-  - downloads/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}/bin/mongod --dbpath var/db --replSet rs0 --fork --logpath var/log/mongod.log
-  - sleep 10
-  - |-
-    downloads/mongodb-linux-x86_64-ubuntu1604-${MONGO_VERSION}/bin/mongo --eval "rs.initiate({_id: 'rs0', members:[{_id: 0, host: '127.0.0.1:27017'}]});"
-    sleep 15
-
-jdk:
-  - openjdk13
-  - openjdk-ea
-
-matrix:
-  allow_failures:
-    - jdk: openjdk-ea
-
-env:
-  matrix:
-    - MONGO_VERSION=4.2.0
-    - MONGO_VERSION=4.0.14
-    - MONGO_VERSION=3.6.16
-  global:
-    - PROFILE=ci
-
-sudo: false
-
-cache:
-  directories:
-    - $HOME/.m2
-    - downloads
-
-install: true
-
-script:
-  - "./mvnw clean dependency:list test -Pjava11 -Dsort -U"

Jenkinsfile

@@ -3,7 +3,7 @@ pipeline {
 
 	triggers {
 		pollSCM 'H/10 * * * *'
-		upstream(upstreamProjects: "spring-data-commons/master", threshold: hudson.model.Result.SUCCESS)
+		upstream(upstreamProjects: "spring-data-commons/2.3.x", threshold: hudson.model.Result.SUCCESS)
 	}
 
 	options {
@@ -46,16 +46,16 @@ pipeline {
 						}
 					}
 				}
-				stage('Publish JDK 13 + MongoDB 4.2') {
+				stage('Publish JDK 14 + MongoDB 4.2') {
 					when {
-						changeset "ci/openjdk13-mongodb-4.2/**"
+						changeset "ci/openjdk14-mongodb-4.2/**"
 					}
 					agent { label 'data' }
 					options { timeout(time: 30, unit: 'MINUTES') }
 
 					steps {
 						script {
-							def image = docker.build("springci/spring-data-openjdk13-with-mongodb-4.2.0", "ci/openjdk13-mongodb-4.2/")
+							def image = docker.build("springci/spring-data-openjdk14-with-mongodb-4.2.0", "ci/openjdk14-mongodb-4.2/")
 							docker.withRegistry('', 'hub.docker.com-springbuildmaster') {
 								image.push()
 							}
@@ -68,7 +68,7 @@ pipeline {
 		stage("test: baseline (jdk8)") {
 			when {
 				anyOf {
-					branch 'master'
+					branch '3.0.x'
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -94,7 +94,7 @@ pipeline {
 		stage("Test other configurations") {
 			when {
 				anyOf {
-					branch 'master'
+					branch '3.0.x'
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -139,10 +139,10 @@ pipeline {
 					}
 				}
 
-				stage("test: baseline (jdk13)") {
+				stage("test: baseline (jdk14)") {
 					agent {
 						docker {
-							image 'springci/spring-data-openjdk13-with-mongodb-4.2.0:latest'
+							image 'springci/spring-data-openjdk14-with-mongodb-4.2.0:latest'
 							label 'data'
 							args '-v $HOME:/tmp/jenkins-home'
 						}
@@ -164,7 +164,7 @@ pipeline {
 		stage('Release to artifactory') {
 			when {
 				anyOf {
-					branch 'master'
+					branch '3.0.x'
 					not { triggeredBy 'UpstreamCause' }
 				}
 			}
@@ -196,7 +196,7 @@ pipeline {
 
 		stage('Publish documentation') {
 			when {
-				branch 'master'
+				branch '3.0.x'
 			}
 			agent {
 				docker {

README.adoc

@@ -52,11 +52,6 @@ public class MyService {
 @EnableMongoRepositories
 class ApplicationConfig extends AbstractMongoClientConfiguration {
 
-  @Override
-  public MongoClient mongoClient() {
-    return MongoClients.create();
-  }
-
   @Override
   protected String getDatabaseName() {
     return "springdata";

ci/openjdk14-mongodb-4.2/Dockerfile

@@ -1,4 +1,4 @@
-FROM adoptopenjdk/openjdk13:latest
+FROM adoptopenjdk/openjdk14:latest
 
 ENV TZ=Etc/UTC
 ENV DEBIAN_FRONTEND=noninteractive

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>3.0.0.RC1</version>
+	<version>3.0.4.RELEASE</version>
 	<packaging>pom</packaging>
 
 	<name>Spring Data MongoDB</name>
@@ -15,7 +15,7 @@
 	<parent>
 		<groupId>org.springframework.data.build</groupId>
 		<artifactId>spring-data-parent</artifactId>
-		<version>2.3.0.RC1</version>
+		<version>2.3.4.RELEASE</version>
 	</parent>
 
 	<modules>
@@ -26,8 +26,8 @@
 	<properties>
 		<project.type>multi</project.type>
 		<dist.id>spring-data-mongodb</dist.id>
-		<springdata.commons>2.3.0.RC1</springdata.commons>
-		<mongo>4.0.1</mongo>
+		<springdata.commons>2.3.4.RELEASE</springdata.commons>
+		<mongo>4.0.5</mongo>
 		<mongo.reactivestreams>${mongo}</mongo.reactivestreams>
 		<jmh.version>1.19</jmh.version>
 	</properties>
@@ -134,8 +134,8 @@
 
 	<repositories>
 		<repository>
-			<id>spring-libs-milestone</id>
-			<url>https://repo.spring.io/libs-milestone</url>
+			<id>spring-libs-release</id>
+			<url>https://repo.spring.io/libs-release</url>
 		</repository>
 		<repository>
 			<id>sonatype-libs-snapshot</id>

spring-data-mongodb-benchmarks/pom.xml

@@ -7,7 +7,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.0.0.RC1</version>
+		<version>3.0.4.RELEASE</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb-distribution/pom.xml

@@ -14,7 +14,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.0.0.RC1</version>
+		<version>3.0.4.RELEASE</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/pom.xml

@@ -11,7 +11,7 @@
 	<parent>
 		<groupId>org.springframework.data</groupId>
 		<artifactId>spring-data-mongodb-parent</artifactId>
-		<version>3.0.0.RC1</version>
+		<version>3.0.4.RELEASE</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/LazyLoadingException.java

@@ -25,8 +25,10 @@ public class LazyLoadingException extends UncategorizedDataAccessException {
 	private static final long serialVersionUID = -7089224903873220037L;
 
 	/**
-	 * @param msg
-	 * @param cause
+	 * Constructor for LazyLoadingException.
+	 *
+	 * @param msg the detail message.
+	 * @param cause the exception thrown by underlying data access API.
 	 */
 	public LazyLoadingException(String msg, Throwable cause) {
 		super(msg, cause);

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoDatabaseFactory.java

@@ -45,7 +45,7 @@ public interface MongoDatabaseFactory extends CodecRegistryProvider, MongoSessio
 	/**
 	 * Obtain a {@link MongoDatabase} instance to access the database with the given name.
 	 *
-	 * @param dbName
+	 * @param dbName must not be {@literal null}.
 	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoDatabaseUtils.java

@@ -79,7 +79,7 @@ public class MongoDatabaseUtils {
 	 * @param factory the {@link MongoDatabaseFactory} to get the {@link MongoDatabase} from.
 	 * @return the {@link MongoDatabase} that is potentially associated with a transactional {@link ClientSession}.
 	 */
-	public static MongoDatabase getDatabase(String dbName, MongoDatabaseFactory factory) {
+	public static MongoDatabase getDatabase(@Nullable String dbName, MongoDatabaseFactory factory) {
 		return doGetMongoDatabase(dbName, factory, SessionSynchronization.ON_ACTUAL_TRANSACTION);
 	}
 
@@ -94,7 +94,7 @@ public class MongoDatabaseUtils {
 	 * @param sessionSynchronization the synchronization to use. Must not be {@literal null}.
 	 * @return the {@link MongoDatabase} that is potentially associated with a transactional {@link ClientSession}.
 	 */
-	public static MongoDatabase getDatabase(String dbName, MongoDatabaseFactory factory,
+	public static MongoDatabase getDatabase(@Nullable String dbName, MongoDatabaseFactory factory,
 			SessionSynchronization sessionSynchronization) {
 		return doGetMongoDatabase(dbName, factory, sessionSynchronization);
 	}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/MongoDbFactory.java

@@ -33,7 +33,7 @@ public interface MongoDbFactory extends MongoDatabaseFactory {
 	/**
 	 * Creates a default {@link MongoDatabase} instance.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 * @deprecated since 3.0. Use {@link #getMongoDatabase()} instead.
 	 */
@@ -46,7 +46,7 @@ public interface MongoDbFactory extends MongoDatabaseFactory {
 	 * Obtain a {@link MongoDatabase} instance to access the database with the given name.
 	 *
 	 * @param dbName must not be {@literal null} or empty.
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 * @deprecated since 3.0. Use {@link #getMongoDatabase(String)} instead.
 	 */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/ReactiveMongoDatabaseFactory.java

@@ -31,6 +31,7 @@ import com.mongodb.reactivestreams.client.MongoDatabase;
  *
  * @author Mark Paluch
  * @author Christoph Strobl
+ * @author Mathieu Ouellet
  * @since 2.0
  */
 public interface ReactiveMongoDatabaseFactory extends CodecRegistryProvider {
@@ -38,19 +39,19 @@ public interface ReactiveMongoDatabaseFactory extends CodecRegistryProvider {
 	/**
 	 * Creates a default {@link MongoDatabase} instance.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 */
-	MongoDatabase getMongoDatabase() throws DataAccessException;
+	Mono<MongoDatabase> getMongoDatabase() throws DataAccessException;
 
 	/**
 	 * Obtain a {@link MongoDatabase} instance to access the database with the given name.
 	 *
 	 * @param dbName must not be {@literal null} or empty.
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws DataAccessException
 	 */
-	MongoDatabase getMongoDatabase(String dbName) throws DataAccessException;
+	Mono<MongoDatabase> getMongoDatabase(String dbName) throws DataAccessException;
 
 	/**
 	 * Exposes a shared {@link MongoExceptionTranslator}.
@@ -64,10 +65,7 @@ public interface ReactiveMongoDatabaseFactory extends CodecRegistryProvider {
 	 *
 	 * @return never {@literal null}.
 	 */
-	@Override
-	default CodecRegistry getCodecRegistry() {
-		return getMongoDatabase().getCodecRegistry();
-	}
+	CodecRegistry getCodecRegistry();
 
 	/**
 	 * Obtain a {@link Mono} emitting a {@link ClientSession} for given {@link ClientSessionOptions options}.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/ReactiveMongoDatabaseUtils.java

@@ -41,6 +41,7 @@ import com.mongodb.reactivestreams.client.MongoDatabase;
  *
  * @author Mark Paluch
  * @author Christoph Strobl
+ * @author Mathieu Ouellet
  * @since 2.2
  */
 public class ReactiveMongoDatabaseUtils {
@@ -142,14 +143,13 @@ public class ReactiveMongoDatabaseUtils {
 				.flatMap(synchronizationManager -> {
 
 					return doGetSession(synchronizationManager, factory, sessionSynchronization) //
-							.map(it -> getMongoDatabaseOrDefault(dbName, factory.withSession(it)));
-				})
-				.onErrorResume(NoTransactionException.class,
-						e -> Mono.fromSupplier(() -> getMongoDatabaseOrDefault(dbName, factory)))
-				.defaultIfEmpty(getMongoDatabaseOrDefault(dbName, factory));
+							.flatMap(it -> getMongoDatabaseOrDefault(dbName, factory.withSession(it)));
+				}) //
+				.onErrorResume(NoTransactionException.class, e -> getMongoDatabaseOrDefault(dbName, factory))
+				.switchIfEmpty(getMongoDatabaseOrDefault(dbName, factory));
 	}
 
-	private static MongoDatabase getMongoDatabaseOrDefault(@Nullable String dbName,
+	private static Mono<MongoDatabase> getMongoDatabaseOrDefault(@Nullable String dbName,
 			ReactiveMongoDatabaseFactory factory) {
 		return StringUtils.hasText(dbName) ? factory.getMongoDatabase(dbName) : factory.getMongoDatabase();
 	}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/config/EnableMongoAuditing.java

@@ -42,21 +42,21 @@ public @interface EnableMongoAuditing {
 	/**
 	 * Configures the {@link AuditorAware} bean to be used to lookup the current principal.
 	 *
-	 * @return
+	 * @return empty {@link String} by default.
 	 */
 	String auditorAwareRef() default "";
 
 	/**
 	 * Configures whether the creation and modification dates are set. Defaults to {@literal true}.
 	 *
-	 * @return
+	 * @return {@literal true} by default.
 	 */
 	boolean setDates() default true;
 
 	/**
 	 * Configures whether the entity shall be marked as modified on creation. Defaults to {@literal true}.
 	 *
-	 * @return
+	 * @return {@literal true} by default.
 	 */
 	boolean modifyOnCreate() default true;
 
@@ -64,7 +64,7 @@ public @interface EnableMongoAuditing {
 	 * Configures a {@link DateTimeProvider} bean name that allows customizing the {@link org.joda.time.DateTime} to be
 	 * used for setting creation and modification dates.
 	 *
-	 * @return
+	 * @return empty {@link String} by default.
 	 */
 	String dateTimeProviderRef() default "";
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/config/MongoConfigurationSupport.java

@@ -95,7 +95,7 @@ public abstract class MongoConfigurationSupport {
 	/**
 	 * Register custom {@link Converter}s in a {@link CustomConversions} object if required. These
 	 * {@link CustomConversions} will be registered with the
-	 * {@link org.springframework.data.mongodb.core.convert.MappingMongoConverter} and {@link #mongoMappingContext()}.
+	 * {@link org.springframework.data.mongodb.core.convert.MappingMongoConverter} and {@link MongoMappingContext}.
 	 * Returns an empty {@link MongoCustomConversions} instance by default.
 	 * <p>
 	 * <strong>NOTE:</strong> Use {@link #configureConverters(MongoConverterConfigurationAdapter)} to configure MongoDB
@@ -217,6 +217,7 @@ public abstract class MongoConfigurationSupport {
 	protected MongoClientSettings mongoClientSettings() {
 
 		MongoClientSettings.Builder builder = MongoClientSettings.builder();
+		builder.uuidRepresentation(UuidRepresentation.JAVA_LEGACY);
 		configureClientSettings(builder);
 		return builder.build();
 	}
@@ -228,6 +229,6 @@ public abstract class MongoConfigurationSupport {
 	 * @since 3.0
 	 */
 	protected void configureClientSettings(MongoClientSettings.Builder builder) {
-		builder.uuidRepresentation(UuidRepresentation.JAVA_LEGACY);
+		// customization hook
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/BulkOperations.java

@@ -94,7 +94,6 @@ public interface BulkOperations {
 	 * Add a list of updates to the bulk operation. For each update request, all matching documents are updated.
 	 *
 	 * @param updates Update operations to perform.
-	 * @return The bulk operation.
 	 * @return the current {@link BulkOperations} instance with the update added, will never be {@literal null}.
 	 */
 	BulkOperations updateMulti(List<Pair<Query, Update>> updates);
@@ -105,7 +104,6 @@ public interface BulkOperations {
 	 *
 	 * @param query Update criteria.
 	 * @param update Update operation to perform.
-	 * @return The bulk operation.
 	 * @return the current {@link BulkOperations} instance with the update added, will never be {@literal null}.
 	 */
 	BulkOperations upsert(Query query, Update update);
@@ -115,7 +113,6 @@ public interface BulkOperations {
 	 * else an insert.
 	 *
 	 * @param updates Updates/insert operations to perform.
-	 * @return The bulk operation.
 	 * @return the current {@link BulkOperations} instance with the update added, will never be {@literal null}.
 	 */
 	BulkOperations upsert(List<Pair<Query, Update>> updates);

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/CollectionCallback.java

@@ -29,7 +29,7 @@ import com.mongodb.client.MongoCollection;
  * @author Grame Rocher
  * @author Oliver Gierke
  * @author John Brisbin
- * @auhtor Christoph Strobl
+ * @author Christoph Strobl
  * @since 1.0
  */
 public interface CollectionCallback<T> {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/CollectionOptions.java

@@ -95,7 +95,7 @@ public class CollectionOptions {
 
 	/**
 	 * Create new {@link CollectionOptions} with already given settings and capped set to {@literal true}. <br />
-	 * <strong>NOTE</strong> Using capped collections requires defining {@link #size(int)}.
+	 * <strong>NOTE</strong> Using capped collections requires defining {@link #size(long)}.
 	 *
 	 * @return new {@link CollectionOptions}.
 	 * @since 2.0

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/CursorPreparer.java

@@ -43,9 +43,10 @@ public interface CursorPreparer extends ReadPreferenceAware {
 	/**
 	 * Prepare the given cursor (apply limits, skips and so on). Returns the prepared cursor.
 	 *
-	 * @param cursor
+	 * @param iterable must not be {@literal null}.
+	 * @return never {@literal null}.
 	 */
-	FindIterable<Document> prepare(FindIterable<Document> cursor);
+	FindIterable<Document> prepare(FindIterable<Document> iterable);
 
 	/**
 	 * Apply query specific settings to {@link MongoCollection} and initate a find operation returning a
@@ -53,7 +54,7 @@ public interface CursorPreparer extends ReadPreferenceAware {
 	 *
 	 * @param collection must not be {@literal null}.
 	 * @param find must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws IllegalArgumentException if one of the required arguments is {@literal null}.
 	 * @since 2.2
 	 */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/DocumentCallbackHandler.java

@@ -23,7 +23,7 @@ import com.mongodb.MongoException;
 /**
  * An interface used by {@link MongoTemplate} for processing documents returned from a MongoDB query on a per-document
  * basis. Implementations of this interface perform the actual work of prcoessing each document but don't need to worry
- * about exception handling. {@MongoException}s will be caught and translated by the calling MongoTemplate An
+ * about exception handling. {@link MongoException}s will be caught and translated by the calling MongoTemplate An
  * DocumentCallbackHandler is typically stateful: It keeps the result state within the object, to be available later for
  * later inspection.
  *

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/EntityOperations.java

@@ -66,7 +66,7 @@ class EntityOperations {
 	 * Creates a new {@link Entity} for the given bean.
 	 *
 	 * @param entity must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Entity}.
 	 */
 	@SuppressWarnings({ "unchecked", "rawtypes" })
 	public <T> Entity<T> forEntity(T entity) {
@@ -89,7 +89,7 @@ class EntityOperations {
 	 *
 	 * @param entity must not be {@literal null}.
 	 * @param conversionService must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link AdaptibleEntity}.
 	 */
 	@SuppressWarnings({ "unchecked", "rawtypes" })
 	public <T> AdaptibleEntity<T> forEntity(T entity, ConversionService conversionService) {
@@ -108,6 +108,10 @@ class EntityOperations {
 		return AdaptibleMappedEntity.of(entity, context, conversionService);
 	}
 
+	/**
+	 * @param entityClass should not be null.
+	 * @return the {@link MongoPersistentEntity#getCollection() collection name}.
+	 */
 	public String determineCollectionName(@Nullable Class<?> entityClass) {
 
 		if (entityClass == null) {
@@ -138,7 +142,7 @@ class EntityOperations {
 	 * {@code _id} if no identifier property can be found.
 	 *
 	 * @param type must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public String getIdPropertyName(Class<?> type) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ExecutableAggregationOperation.java

@@ -88,9 +88,9 @@ public interface ExecutableAggregationOperation {
 
 		/**
 		 * Apply pipeline operations as specified and stream all matching elements. <br />
-		 * Returns a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.Cursor}
+		 * Returns a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.client.FindIterable}
 		 *
-		 * @return a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.Cursor} that needs to be closed.
+		 * @return a {@link CloseableIterator} that wraps the a Mongo DB {@link com.mongodb.client.FindIterable} that needs to be closed.
 		 *         Never {@literal null}.
 		 */
 		CloseableIterator<T> stream();

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ExecutableFindOperation.java

@@ -44,7 +44,7 @@ import com.mongodb.client.MongoCollection;
  *         query(Human.class)
  *             .inCollection("star-wars")
  *             .as(Jedi.class)
- *             .matching(query(where("firstname").is("luke")))
+ *             .matching(where("firstname").is("luke"))
  *             .all();
  *     </code>
  * </pre>
@@ -118,7 +118,7 @@ public interface ExecutableFindOperation {
 		/**
 		 * Stream all matching elements.
 		 *
-		 * @return a {@link Stream} that wraps the a Mongo DB {@link com.mongodb.Cursor} that needs to be closed. Never
+		 * @return a {@link Stream} that wraps the a Mongo DB {@link com.mongodb.client.FindIterable} that needs to be closed. Never
 		 *         {@literal null}.
 		 */
 		Stream<T> stream();
@@ -174,13 +174,13 @@ public interface ExecutableFindOperation {
 		/**
 		 * Set the filter {@link CriteriaDefinition criteria} to be used.
 		 *
-		 * @param criteriaDefinition must not be {@literal null}.
+		 * @param criteria must not be {@literal null}.
 		 * @return new instance of {@link TerminatingFind}.
-		 * @throws IllegalArgumentException if query is {@literal null}.
+		 * @throws IllegalArgumentException if criteria is {@literal null}.
 		 * @since 3.0
 		 */
-		default TerminatingFind<T> matching(CriteriaDefinition criteriaDefinition) {
-			return matching(Query.query(criteriaDefinition));
+		default TerminatingFind<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
 		}
 
 		/**
@@ -304,9 +304,21 @@ public interface ExecutableFindOperation {
 		 *
 		 * @param query must not be {@literal null}.
 		 * @return new instance of {@link TerminatingDistinct}.
-		 * @throws IllegalArgumentException if resultType is {@literal null}.
+		 * @throws IllegalArgumentException if query is {@literal null}.
 		 */
 		TerminatingDistinct<T> matching(Query query);
+
+		/**
+		 * Set the filter {@link CriteriaDefinition criteria} to be used.
+		 *
+		 * @param criteria must not be {@literal null}.
+		 * @return new instance of {@link TerminatingDistinct}.
+		 * @throws IllegalArgumentException if criteria is {@literal null}.
+		 * @since 3.0
+		 */
+		default TerminatingDistinct<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
+		}
 	}
 
 	/**

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ExecutableFindOperationSupport.java

@@ -257,9 +257,9 @@ class ExecutableFindOperationSupport implements ExecutableFindOperation {
 		 * @see org.springframework.data.mongodb.core.CursorPreparer#prepare(com.mongodb.clientFindIterable)
 		 */
 		@Override
-		public FindIterable<Document> prepare(FindIterable<Document> cursor) {
+		public FindIterable<Document> prepare(FindIterable<Document> iterable) {
 
-			FindIterable<Document> target = delegate != null ? delegate.prepare(cursor) : cursor;
+			FindIterable<Document> target = delegate != null ? delegate.prepare(iterable) : iterable;
 			return limit.map(target::limit).orElse(target);
 		}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ExecutableMapReduceOperation.java

@@ -152,13 +152,13 @@ public interface ExecutableMapReduceOperation {
 		/**
 		 * Set the filter {@link CriteriaDefinition criteria} to be used.
 		 *
-		 * @param criteriaDefinition must not be {@literal null}.
+		 * @param criteria must not be {@literal null}.
 		 * @return new instance of {@link TerminatingMapReduce}.
 		 * @throws IllegalArgumentException if query is {@literal null}.
 		 * @since 3.0
 		 */
-		default TerminatingMapReduce<T> matching(CriteriaDefinition criteriaDefinition) {
-			return matching(Query.query(criteriaDefinition));
+		default TerminatingMapReduce<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
 		}
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ExecutableRemoveOperation.java

@@ -124,13 +124,13 @@ public interface ExecutableRemoveOperation {
 		/**
 		 * Set the filter {@link CriteriaDefinition criteria} to be used.
 		 *
-		 * @param criteriaDefinition must not be {@literal null}.
+		 * @param criteria must not be {@literal null}.
 		 * @return new instance of {@link TerminatingRemove}.
 		 * @throws IllegalArgumentException if query is {@literal null}.
 		 * @since 3.0
 		 */
-		default TerminatingRemove<T> matching(CriteriaDefinition criteriaDefinition) {
-			return matching(Query.query(criteriaDefinition));
+		default TerminatingRemove<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
 		}
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ExecutableUpdateOperation.java

@@ -215,13 +215,13 @@ public interface ExecutableUpdateOperation {
 		/**
 		 * Set the filter {@link CriteriaDefinition criteria} to be used.
 		 *
-		 * @param criteriaDefinition must not be {@literal null}.
+		 * @param criteria must not be {@literal null}.
 		 * @return new instance of {@link UpdateWithUpdate}.
 		 * @throws IllegalArgumentException if query is {@literal null}.
 		 * @since 3.0
 		 */
-		default UpdateWithUpdate<T> matching(CriteriaDefinition criteriaDefinition) {
-			return matching(Query.query(criteriaDefinition));
+		default UpdateWithUpdate<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
 		}
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/FindAndModifyOptions.java

@@ -78,7 +78,7 @@ public class FindAndModifyOptions {
 	}
 
 	/**
-	 * Create new {@link FindAndModifyOptions} based on option of given {@litearl source}.
+	 * Create new {@link FindAndModifyOptions} based on option of given {@literal source}.
 	 *
 	 * @param source can be {@literal null}.
 	 * @return new instance of {@link FindAndModifyOptions}.
@@ -117,8 +117,8 @@ public class FindAndModifyOptions {
 	/**
 	 * Define the {@link Collation} specifying language-specific rules for string comparison.
 	 *
-	 * @param collation
-	 * @return
+	 * @param collation can be {@literal null}.
+	 * @return this.
 	 * @since 2.0
 	 */
 	public FindAndModifyOptions collation(@Nullable Collation collation) {
@@ -142,7 +142,7 @@ public class FindAndModifyOptions {
 	/**
 	 * Get the {@link Collation} specifying language-specific rules for string comparison.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.0
 	 */
 	public Optional<Collation> getCollation() {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/FindAndReplaceOptions.java

@@ -116,7 +116,7 @@ public class FindAndReplaceOptions {
 	/**
 	 * Get the bit indicating to return the replacement document.
 	 *
-	 * @return
+	 * @return {@literal true} if set.
 	 */
 	public boolean isReturnNew() {
 		return returnNew;
@@ -125,7 +125,7 @@ public class FindAndReplaceOptions {
 	/**
 	 * Get the bit indicating if to create a new document if not exists.
 	 *
-	 * @return
+	 * @return {@literal true} if set.
 	 */
 	public boolean isUpsert() {
 		return upsert;

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/FindPublisherPreparer.java

@@ -54,7 +54,7 @@ public interface FindPublisherPreparer extends ReadPreferenceAware {
 	 *
 	 * @param collection must not be {@literal null}.
 	 * @param find must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 * @throws IllegalArgumentException if one of the required arguments is {@literal null}.
 	 * @since 2.2
 	 */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/GeoCommandStatistics.java

@@ -47,7 +47,7 @@ class GeoCommandStatistics {
 	 * Creates a new {@link GeoCommandStatistics} from the given command result extracting the statistics.
 	 *
 	 * @param commandResult must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static GeoCommandStatistics from(Document commandResult) {
 
@@ -61,7 +61,7 @@ class GeoCommandStatistics {
 	 * Returns the average distance reported by the command result. Mitigating a removal of the field in case the command
 	 * didn't return any result introduced in MongoDB 3.2 RC1.
 	 *
-	 * @return
+	 * @return never {@literal null}, uses {@link Double#NaN} if {@literal avgDistance} does not exist.
 	 * @see <a href="https://jira.mongodb.org/browse/SERVER-21024">MongoDB Jira SERVER-21024</a>
 	 */
 	public double getAverageDistance() {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoDatabaseFactorySupport.java

@@ -254,7 +254,7 @@ public abstract class MongoDatabaseFactorySupport<C> implements MongoDatabaseFac
 			factory.addAdvice(new SessionAwareMethodInterceptor<>(session, target, ClientSession.class, MongoDatabase.class,
 					this::proxyDatabase, MongoCollection.class, this::proxyCollection));
 
-			return targetType.cast(factory.getProxy());
+			return targetType.cast(factory.getProxy(target.getClass().getClassLoader()));
 		}
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoOperations.java

@@ -79,7 +79,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	 * The collection name used for the specified class by this template.
 	 *
 	 * @param entityClass must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	String getCollectionName(Class<?> entityClass);
 
@@ -363,7 +363,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Returns the {@link ScriptOperations} that can be performed on {@link com.mongodb.client.MongoDatabase} level.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 1.7
 	 * @deprecated since 2.2. The {@code eval} command has been removed without replacement in MongoDB Server 4.2.0.
 	 */
@@ -1557,7 +1557,7 @@ public interface MongoOperations extends FluentMongoOperations {
 	/**
 	 * Returns the underlying {@link MongoConverter}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	MongoConverter getConverter();
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/MongoTemplate.java

@@ -93,6 +93,7 @@ import org.springframework.data.mongodb.core.mapreduce.GroupBy;
 import org.springframework.data.mongodb.core.mapreduce.GroupByResults;
 import org.springframework.data.mongodb.core.mapreduce.MapReduceOptions;
 import org.springframework.data.mongodb.core.mapreduce.MapReduceResults;
+import org.springframework.data.mongodb.core.query.BasicQuery;
 import org.springframework.data.mongodb.core.query.Collation;
 import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.Meta;
@@ -238,7 +239,8 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		this.projectionFactory = new SpelAwareProxyProjectionFactory();
 		this.operations = new EntityOperations(this.mongoConverter.getMappingContext());
 		this.propertyOperations = new PropertyOperations(this.mongoConverter.getMappingContext());
-		this.queryOperations = new QueryOperations(queryMapper, updateMapper, operations, mongoDbFactory);
+		this.queryOperations = new QueryOperations(queryMapper, updateMapper, operations, propertyOperations,
+				mongoDbFactory);
 
 		// We always have a mapping context in the converter, whether it's a simple one or not
 		mappingContext = this.mongoConverter.getMappingContext();
@@ -424,8 +426,10 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 			MongoPersistentEntity<?> persistentEntity = mappingContext.getPersistentEntity(entityType);
 
-			Document mappedFields = getMappedFieldsObject(query.getFieldsObject(), persistentEntity, returnType);
-			Document mappedQuery = queryMapper.getMappedObject(query.getQueryObject(), persistentEntity);
+			QueryContext queryContext = queryOperations.createQueryContext(query);
+
+			Document mappedQuery = queryContext.getMappedQuery(persistentEntity);
+			Document mappedFields = queryContext.getMappedFields(persistentEntity, returnType, projectionFactory);
 
 			FindIterable<Document> cursor = new QueryCursorPreparer(query, entityType).initiateFind(collection,
 					col -> col.find(mappedQuery, Document.class).projection(mappedFields));
@@ -1054,7 +1058,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		QueryContext queryContext = queryOperations.createQueryContext(query);
 
 		Document mappedQuery = queryContext.getMappedQuery(entity);
-		Document mappedFields = queryContext.getMappedFields(entity);
+		Document mappedFields = queryContext.getMappedFields(entity, resultType, projectionFactory);
 		Document mappedSort = queryContext.getMappedSort(entity);
 
 		replacement = maybeCallBeforeConvert(replacement, collectionName);
@@ -1174,7 +1178,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 	/**
 	 * Prepare the collection before any processing is done using it. This allows a convenient way to apply settings like
-	 * slaveOk() etc. Can be overridden in sub-classes.
+	 * withCodecRegistry() etc. Can be overridden in sub-classes.
 	 *
 	 * @param collection
 	 */
@@ -1819,7 +1823,11 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		if (query.getMeta().getMaxTimeMsec() != null) {
 			mapReduce = mapReduce.maxTime(query.getMeta().getMaxTimeMsec(), TimeUnit.MILLISECONDS);
 		}
-		mapReduce = mapReduce.sort(getMappedSortObject(query, domainType));
+
+		Document mappedSort = getMappedSortObject(query, domainType);
+		if (mappedSort != null && !mappedSort.isEmpty()) {
+			mapReduce = mapReduce.sort(getMappedSortObject(query, domainType));
+		}
 
 		mapReduce = mapReduce
 				.filter(queryMapper.getMappedObject(query.getQueryObject(), mappingContext.getPersistentEntity(domainType)));
@@ -2117,7 +2125,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 			List<Document> rawResult = new ArrayList<>();
 
-			Class<?> domainType = aggregation instanceof TypedAggregation ? ((TypedAggregation) aggregation).getInputType()
+			Class<?> domainType = aggregation instanceof TypedAggregation ? ((TypedAggregation<?>) aggregation).getInputType()
 					: null;
 
 			Optional<Collation> collation = Optionals.firstNonEmpty(options::getCollation,
@@ -2138,6 +2146,17 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 				aggregateIterable = aggregateIterable.maxTime(options.getMaxTime().toMillis(), TimeUnit.MILLISECONDS);
 			}
 
+			if (options.isSkipResults()) {
+
+				// toCollection only allowed for $out and $merge if those are the last stages
+				if (aggregation.getPipeline().isOutOrMerge()) {
+					aggregateIterable.toCollection();
+				} else {
+					aggregateIterable.first();
+				}
+				return new AggregationResults<>(Collections.emptyList(), new Document());
+			}
+
 			MongoIterable<O> iterable = aggregateIterable.map(val -> {
 
 				rawResult.add(val);
@@ -2435,8 +2454,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		MongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(entityClass);
 
-		Document mappedQuery = queryMapper.getMappedObject(query, entity);
-		Document mappedFields = queryMapper.getMappedObject(fields, entity);
+		QueryContext queryContext = queryOperations.createQueryContext(new BasicQuery(query, fields));
+		Document mappedFields = queryContext.getMappedFields(entity, entityClass, projectionFactory);
+		Document mappedQuery = queryContext.getMappedQuery(entity);
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug("findOne using query: {} fields: {} for class: {} in collection: {}", serializeToJsonSafely(query),
@@ -2486,8 +2506,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		MongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(entityClass);
 
-		Document mappedFields = queryMapper.getMappedFields(fields, entity);
-		Document mappedQuery = queryMapper.getMappedObject(query, entity);
+		QueryContext queryContext = queryOperations.createQueryContext(new BasicQuery(query, fields));
+		Document mappedFields = queryContext.getMappedFields(entity, entityClass, projectionFactory);
+		Document mappedQuery = queryContext.getMappedQuery(entity);
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug("find using query: {} fields: {} for class: {} in collection: {}",
@@ -2509,8 +2530,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		MongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(sourceClass);
 
-		Document mappedFields = getMappedFieldsObject(fields, entity, targetClass);
-		Document mappedQuery = queryMapper.getMappedObject(query, entity);
+		QueryContext queryContext = queryOperations.createQueryContext(new BasicQuery(query, fields));
+		Document mappedFields = queryContext.getMappedFields(entity, targetClass, projectionFactory);
+		Document mappedQuery = queryContext.getMappedQuery(entity);
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug("find using query: {} fields: {} for class: {} in collection: {}",
@@ -2839,23 +2861,6 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		return queryMapper.getMappedSort(query.getSortObject(), mappingContext.getPersistentEntity(type));
 	}
 
-	private Document getMappedFieldsObject(Document fields, @Nullable MongoPersistentEntity<?> entity,
-			Class<?> targetType) {
-
-		if (entity == null) {
-			return fields;
-		}
-
-		Document projectedFields = propertyOperations.computeFieldsForProjection(projectionFactory, fields,
-				entity.getType(), targetType);
-
-		if (ObjectUtils.nullSafeEquals(fields, projectedFields)) {
-			return queryMapper.getMappedFields(projectedFields, entity);
-		}
-
-		return queryMapper.getMappedFields(projectedFields, mappingContext.getRequiredPersistentEntity(targetType));
-	}
-
 	/**
 	 * Tries to convert the given {@link RuntimeException} into a {@link DataAccessException} but returns the original
 	 * exception if the conversation failed. Thus allows safe re-throwing of the return value.
@@ -3129,12 +3134,13 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		@Nullable
 		public T doWith(@Nullable Document document) {
 
+			T source = null;
+
 			if (document != null) {
 				maybeEmitEvent(new AfterLoadEvent<>(document, type, collectionName));
+				source = reader.read(type, document);
 			}
 
-			T source = reader.read(type, document);
-
 			if (source != null) {
 				maybeEmitEvent(new AfterConvertEvent<>(document, source, collectionName));
 				source = maybeCallAfterConvert(source, document, collectionName);
@@ -3175,9 +3181,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			Class<?> typeToRead = targetType.isInterface() || targetType.isAssignableFrom(entityType) ? entityType
 					: targetType;
 
-			if (document != null) {
-				maybeEmitEvent(new AfterLoadEvent<>(document, targetType, collectionName));
-			}
+			maybeEmitEvent(new AfterLoadEvent<>(document, targetType, collectionName));
 
 			Object source = reader.read(typeToRead, document);
 			Object result = targetType.isInterface() ? projectionFactory.createProjection(targetType, source) : source;
@@ -3206,9 +3210,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.core.CursorPreparer#prepare(com.mongodb.DBCursor)
 		 */
-		public FindIterable<Document> prepare(FindIterable<Document> cursor) {
+		public FindIterable<Document> prepare(FindIterable<Document> iterable) {
 
-			FindIterable<Document> cursorToUse = cursor;
+			FindIterable<Document> cursorToUse = iterable;
 
 			operations.forType(type).getCollation(query) //
 					.map(Collation::toMongoCollation) //
@@ -3267,6 +3271,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 							case PARTIAL:
 								cursorToUse = cursorToUse.partial(true);
 								break;
+							case SECONDARY_READS:
 							case SLAVE_OK:
 								break;
 							default:
@@ -3284,7 +3289,8 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		@Override
 		public ReadPreference getReadPreference() {
-			return query.getMeta().getFlags().contains(CursorOption.SLAVE_OK) ? ReadPreference.primaryPreferred() : null;
+			return (query.getMeta().getFlags().contains(CursorOption.SECONDARY_READS)
+					|| query.getMeta().getFlags().contains(CursorOption.SLAVE_OK)) ? ReadPreference.primaryPreferred() : null;
 		}
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/QueryOperations.java

@@ -47,8 +47,10 @@ import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.data.mongodb.core.query.UpdateDefinition.ArrayFilter;
 import org.springframework.data.mongodb.util.BsonUtils;
+import org.springframework.data.projection.ProjectionFactory;
 import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
 import com.mongodb.client.model.CountOptions;
@@ -70,6 +72,7 @@ class QueryOperations {
 	private final QueryMapper queryMapper;
 	private final UpdateMapper updateMapper;
 	private final EntityOperations entityOperations;
+	private final PropertyOperations propertyOperations;
 	private final CodecRegistryProvider codecRegistryProvider;
 	private final MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext;
 	private final AggregationUtil aggregationUtil;
@@ -81,14 +84,16 @@ class QueryOperations {
 	 * @param queryMapper must not be {@literal null}.
 	 * @param updateMapper must not be {@literal null}.
 	 * @param entityOperations must not be {@literal null}.
+	 * @param propertyOperations must not be {@literal null}.
 	 * @param codecRegistryProvider must not be {@literal null}.
 	 */
 	QueryOperations(QueryMapper queryMapper, UpdateMapper updateMapper, EntityOperations entityOperations,
-			CodecRegistryProvider codecRegistryProvider) {
+			PropertyOperations propertyOperations, CodecRegistryProvider codecRegistryProvider) {
 
 		this.queryMapper = queryMapper;
 		this.updateMapper = updateMapper;
 		this.entityOperations = entityOperations;
+		this.propertyOperations = propertyOperations;
 		this.codecRegistryProvider = codecRegistryProvider;
 		this.mappingContext = queryMapper.getMappingContext();
 		this.aggregationUtil = new AggregationUtil(queryMapper, mappingContext);
@@ -250,14 +255,31 @@ class QueryOperations {
 			return queryMapper.getMappedObject(getQueryObject(), entity);
 		}
 
-		/**
-		 * Get the already mapped {@link Query#getFieldsObject() fields projection}
-		 *
-		 * @param entity the Entity to map field names to. Can be {@literal null}.
-		 * @return never {@literal null}.
-		 */
-		Document getMappedFields(@Nullable MongoPersistentEntity<?> entity) {
-			return queryMapper.getMappedFields(query.getFieldsObject(), entity);
+		Document getMappedFields(@Nullable MongoPersistentEntity<?> entity, Class<?> targetType,
+				ProjectionFactory projectionFactory) {
+
+			Document fields = query.getFieldsObject();
+			Document mappedFields = fields;
+
+			if (entity == null) {
+				return mappedFields;
+			}
+
+			Document projectedFields = propertyOperations.computeFieldsForProjection(projectionFactory, fields,
+					entity.getType(), targetType);
+
+			if (ObjectUtils.nullSafeEquals(fields, projectedFields)) {
+				mappedFields = queryMapper.getMappedFields(projectedFields, entity);
+			} else {
+				mappedFields = queryMapper.getMappedFields(projectedFields,
+						mappingContext.getRequiredPersistentEntity(targetType));
+			}
+
+			if (entity != null && entity.hasTextScoreProperty() && !query.getQueryObject().containsKey("$text")) {
+				mappedFields.remove(entity.getTextScoreProperty().getFieldName());
+			}
+
+			return mappedFields;
 		}
 
 		/**
@@ -319,6 +341,10 @@ class QueryOperations {
 		}
 
 		@Override
+		Document getMappedFields(@Nullable MongoPersistentEntity<?> entity, Class<?> targetType, ProjectionFactory projectionFactory) {
+			return getMappedFields(entity);
+		}
+
 		Document getMappedFields(@Nullable MongoPersistentEntity<?> entity) {
 			return queryMapper.getMappedFields(new Document(fieldName, 1), entity);
 		}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveFindOperation.java

@@ -39,13 +39,14 @@ import org.springframework.data.mongodb.core.query.Query;
  *         query(Human.class)
  *             .inCollection("star-wars")
  *             .as(Jedi.class)
- *             .matching(query(where("firstname").is("luke")))
+ *             .matching(where("firstname").is("luke"))
  *             .all();
  *     </code>
  * </pre>
  *
  * @author Mark Paluch
  * @author Christoph Strobl
+ * @author Juergen Zimmermann
  * @since 2.0
  */
 public interface ReactiveFindOperation {
@@ -148,13 +149,13 @@ public interface ReactiveFindOperation {
 		/**
 		 * Set the filter {@link CriteriaDefinition criteria} to be used.
 		 *
-		 * @param criteriaDefinition must not be {@literal null}.
+		 * @param criteria must not be {@literal null}.
 		 * @return new instance of {@link TerminatingFind}.
-		 * @throws IllegalArgumentException if query is {@literal null}.
+		 * @throws IllegalArgumentException if criteria is {@literal null}.
 		 * @since 3.0
 		 */
-		default TerminatingFind<T> matching(CriteriaDefinition criteriaDefinition) {
-			return matching(Query.query(criteriaDefinition));
+		default TerminatingFind<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
 		}
 
 		/**
@@ -272,9 +273,21 @@ public interface ReactiveFindOperation {
 		 *
 		 * @param query must not be {@literal null}.
 		 * @return new instance of {@link TerminatingDistinct}.
-		 * @throws IllegalArgumentException if resultType is {@literal null}.
+		 * @throws IllegalArgumentException if query is {@literal null}.
 		 */
 		TerminatingDistinct<T> matching(Query query);
+
+		/**
+		 * Set the filter {@link CriteriaDefinition criteria} to be used.
+		 *
+		 * @param criteria must not be {@literal null}.
+		 * @return new instance of {@link TerminatingDistinct}.
+		 * @throws IllegalArgumentException if criteria is {@literal null}.
+		 * @since 3.0
+		 */
+		default TerminatingDistinct<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
+		}
 	}
 
 	/**

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveMapReduceOperation.java

@@ -151,13 +151,13 @@ public interface ReactiveMapReduceOperation {
 		/**
 		 * Set the filter {@link CriteriaDefinition criteria} to be used.
 		 *
-		 * @param criteriaDefinition must not be {@literal null}.
+		 * @param criteria must not be {@literal null}.
 		 * @return new instance of {@link TerminatingMapReduce}.
 		 * @throws IllegalArgumentException if query is {@literal null}.
 		 * @since 3.0
 		 */
-		default TerminatingMapReduce<T> matching(CriteriaDefinition criteriaDefinition) {
-			return matching(Query.query(criteriaDefinition));
+		default TerminatingMapReduce<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
 		}
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveMongoOperations.java

@@ -66,6 +66,7 @@ import com.mongodb.reactivestreams.client.MongoCollection;
  *
  * @author Mark Paluch
  * @author Christoph Strobl
+ * @author Mathieu Ouellet
  * @since 2.0
  * @see Flux
  * @see Mono
@@ -298,7 +299,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * @param collectionName name of the collection.
 	 * @return an existing collection or one created on first server interaction.
 	 */
-	MongoCollection<Document> getCollection(String collectionName);
+	Mono<MongoCollection<Document>> getCollection(String collectionName);
 
 	/**
 	 * Check to see if a collection with a name indicated by the entity class exists.
@@ -1238,7 +1239,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * domain type information. Use {@link #updateFirst(Query, UpdateDefinition, Class, String)} to get full type specific
 	 * support. <br />
 	 * <strong>NOTE:</strong> {@link Query#getSortObject() sorting} is not supported by {@code db.collection.updateOne}.
-	 * Use {@link #findAndModify(Query, Update, Class, String)} instead.
+	 * Use {@link #findAndModify(Query, UpdateDefinition, Class, String)} instead.
 	 *
 	 * @param query the query document that specifies the criteria used to select a record to be updated. Must not be
 	 *          {@literal null}.
@@ -1571,7 +1572,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	/**
 	 * Returns the underlying {@link MongoConverter}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	MongoConverter getConverter();
 
@@ -1579,7 +1580,7 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 * The collection name used for the specified class by this template.
 	 *
 	 * @param entityClass must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.1
 	 */
 	String getCollectionName(Class<?> entityClass);

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveMongoTemplate.java

@@ -94,6 +94,7 @@ import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.data.mongodb.core.mapping.MongoSimpleTypes;
 import org.springframework.data.mongodb.core.mapping.event.*;
 import org.springframework.data.mongodb.core.mapreduce.MapReduceOptions;
+import org.springframework.data.mongodb.core.query.BasicQuery;
 import org.springframework.data.mongodb.core.query.Collation;
 import org.springframework.data.mongodb.core.query.Meta;
 import org.springframework.data.mongodb.core.query.Meta.CursorOption;
@@ -156,6 +157,7 @@ import com.mongodb.reactivestreams.client.MongoDatabase;
  * @author Mark Paluch
  * @author Christoph Strobl
  * @author Roman Puchkovskiy
+ * @author Mathieu Ouellet
  * @since 2.0
  */
 public class ReactiveMongoTemplate implements ReactiveMongoOperations, ApplicationContextAware {
@@ -258,7 +260,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		this.mappingContext = this.mongoConverter.getMappingContext();
 		this.operations = new EntityOperations(this.mappingContext);
 		this.propertyOperations = new PropertyOperations(this.mappingContext);
-		this.queryOperations = new QueryOperations(queryMapper, updateMapper, operations, mongoDatabaseFactory);
+		this.queryOperations = new QueryOperations(queryMapper, updateMapper, operations, propertyOperations,
+				mongoDatabaseFactory);
 
 		// We create indexes based on mapping events
 		if (this.mappingContext instanceof MongoMappingContext) {
@@ -317,8 +320,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	/**
 	 * Configures the {@link WriteConcern} to be used with the template. If none is configured the {@link WriteConcern}
-	 * configured on the {@link MongoDatabaseFactory} will apply. If you configured a {@link Mongo} instance no
-	 * {@link WriteConcern} will be used.
+	 * configured on the {@link MongoDatabaseFactory} will apply.
 	 *
 	 * @param writeConcern can be {@literal null}.
 	 */
@@ -717,15 +719,11 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#getCollection(java.lang.String)
 	 */
-	public MongoCollection<Document> getCollection(String collectionName) {
+	public Mono<MongoCollection<Document>> getCollection(String collectionName) {
 
 		Assert.notNull(collectionName, "Collection name must not be null!");
 
-		try {
-			return this.mongoDatabaseFactory.getMongoDatabase().getCollection(collectionName);
-		} catch (RuntimeException e) {
-			throw potentiallyConvertRuntimeException(e, exceptionTranslator);
-		}
+		return createMono(db -> Mono.just(db.getCollection(collectionName)));
 	}
 
 	/*
@@ -776,7 +774,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		return createFlux(MongoDatabase::listCollectionNames);
 	}
 
-	public MongoDatabase getMongoDatabase() {
+	public Mono<MongoDatabase> getMongoDatabase() {
 		return mongoDatabaseFactory.getMongoDatabase();
 	}
 
@@ -1019,11 +1017,14 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		}
 
 		ReadDocumentCallback<O> readCallback = new ReadDocumentCallback<>(mongoConverter, outputType, collectionName);
-		return execute(collectionName, collection -> aggregateAndMap(collection, pipeline, options, readCallback,
-				aggregation instanceof TypedAggregation ? ((TypedAggregation) aggregation).getInputType() : null));
+		return execute(collectionName,
+				collection -> aggregateAndMap(collection, pipeline, aggregation.getPipeline().isOutOrMerge(), options,
+						readCallback,
+						aggregation instanceof TypedAggregation ? ((TypedAggregation<?>) aggregation).getInputType() : null));
 	}
 
 	private <O> Flux<O> aggregateAndMap(MongoCollection<Document> collection, List<Document> pipeline,
+			boolean isOutOrMerge,
 			AggregationOptions options, ReadDocumentCallback<O> readCallback, @Nullable Class<?> inputType) {
 
 		AggregatePublisher<Document> cursor = collection.aggregate(pipeline, Document.class)
@@ -1043,6 +1044,10 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			cursor = cursor.maxTime(options.getMaxTime().toMillis(), TimeUnit.MILLISECONDS);
 		}
 
+		if (options.isSkipResults()) {
+			return (isOutOrMerge ? Flux.from(cursor.toCollection()) : Flux.from(cursor.first())).thenMany(Mono.empty());
+		}
+
 		return Flux.from(cursor).concatMap(readCallback::doWith);
 	}
 
@@ -1161,7 +1166,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		QueryContext queryContext = queryOperations.createQueryContext(query);
 
 		Document mappedQuery = queryContext.getMappedQuery(entity);
-		Document mappedFields = queryContext.getMappedFields(entity);
+		Document mappedFields = queryContext.getMappedFields(entity, resultType, projectionFactory);
 		Document mappedSort = queryContext.getMappedSort(entity);
 
 		return Mono.defer(() -> {
@@ -2073,24 +2078,25 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		FullDocument fullDocument = ClassUtils.isAssignable(Document.class, targetType) ? FullDocument.DEFAULT
 				: FullDocument.UPDATE_LOOKUP;
 
-		MongoDatabase db = StringUtils.hasText(database) ? mongoDatabaseFactory.getMongoDatabase(database)
-				: getMongoDatabase();
+		return ReactiveMongoDatabaseUtils.getDatabase(database, mongoDatabaseFactory) //
+				.map(db -> {
+					ChangeStreamPublisher<Document> publisher;
+					if (StringUtils.hasText(collectionName)) {
+						publisher = filter.isEmpty() ? db.getCollection(collectionName).watch(Document.class)
+								: db.getCollection(collectionName).watch(filter, Document.class);
 
-		ChangeStreamPublisher<Document> publisher;
-		if (StringUtils.hasText(collectionName)) {
-			publisher = filter.isEmpty() ? db.getCollection(collectionName).watch(Document.class)
-					: db.getCollection(collectionName).watch(filter, Document.class);
+					} else {
+						publisher = filter.isEmpty() ? db.watch(Document.class) : db.watch(filter, Document.class);
+					}
 
-		} else {
-			publisher = filter.isEmpty() ? db.watch(Document.class) : db.watch(filter, Document.class);
-		}
-
-		publisher = options.getResumeToken().map(BsonValue::asDocument).map(publisher::resumeAfter).orElse(publisher);
-		publisher = options.getCollation().map(Collation::toMongoCollation).map(publisher::collation).orElse(publisher);
-		publisher = options.getResumeBsonTimestamp().map(publisher::startAtOperationTime).orElse(publisher);
-		publisher = publisher.fullDocument(options.getFullDocumentLookup().orElse(fullDocument));
-
-		return Flux.from(publisher).map(document -> new ChangeStreamEvent<>(document, targetType, getConverter()));
+					publisher = options.getResumeToken().map(BsonValue::asDocument).map(publisher::resumeAfter).orElse(publisher);
+					publisher = options.getCollation().map(Collation::toMongoCollation).map(publisher::collation)
+							.orElse(publisher);
+					publisher = options.getResumeBsonTimestamp().map(publisher::startAtOperationTime).orElse(publisher);
+					return publisher.fullDocument(options.getFullDocumentLookup().orElse(fullDocument));
+				}) //
+				.flatMapMany(publisher -> Flux.from(publisher)
+						.map(document -> new ChangeStreamEvent<>(document, targetType, getConverter())));
 	}
 
 	List<Document> prepareFilter(ChangeStreamOptions options) {
@@ -2152,7 +2158,11 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			MapReducePublisher<Document> publisher = collection.mapReduce(mapFunction, reduceFunction, Document.class);
 
 			publisher.filter(mappedQuery);
-			publisher.sort(getMappedSortObject(filterQuery, domainType));
+
+			Document mappedSort = getMappedSortObject(filterQuery, domainType);
+			if (mappedSort != null && !mappedSort.isEmpty()) {
+				publisher.sort(mappedSort);
+			}
 
 			if (filterQuery.getMeta().getMaxTimeMsec() != null) {
 				publisher.maxTime(filterQuery.getMeta().getMaxTimeMsec(), TimeUnit.MILLISECONDS);
@@ -2332,7 +2342,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 				LOGGER.debug("Created collection [{}]", collectionName);
 			}
 
-		}).thenReturn(getCollection(collectionName));
+		}).then(getCollection(collectionName));
 	}
 
 	/**
@@ -2369,8 +2379,11 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			Class<T> entityClass, FindPublisherPreparer preparer) {
 
 		MongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(entityClass);
-		Document mappedQuery = queryMapper.getMappedObject(query, entity);
-		Document mappedFields = fields == null ? null : queryMapper.getMappedObject(fields, entity);
+
+		QueryContext queryContext = queryOperations
+				.createQueryContext(new BasicQuery(query, fields != null ? fields : new Document()));
+		Document mappedFields = queryContext.getMappedFields(entity, entityClass, projectionFactory);
+		Document mappedQuery = queryContext.getMappedQuery(entity);
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug(String.format("findOne using query: %s fields: %s for class: %s in collection: %s",
@@ -2405,7 +2418,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * @param query the query document that specifies the criteria used to find a record.
 	 * @param fields the document that specifies the fields to be returned.
 	 * @param entityClass the parameterized type of the returned list.
-	 * @param preparer allows for customization of the {@link DBCursor} used when iterating over the result set, (apply
+	 * @param preparer allows for customization of the {@link com.mongodb.client.FindIterable} used when iterating over the result set, (apply
 	 *          limits, skips and so on).
 	 * @return the {@link List} of converted objects.
 	 */
@@ -2420,8 +2433,9 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		MongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(entityClass);
 
-		Document mappedFields = queryMapper.getMappedFields(fields, entity);
-		Document mappedQuery = queryMapper.getMappedObject(query, entity);
+		QueryContext queryContext = queryOperations.createQueryContext(new BasicQuery(query, fields));
+		Document mappedFields = queryContext.getMappedFields(entity, entityClass, projectionFactory);
+		Document mappedQuery = queryContext.getMappedQuery(entity);
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug(String.format("find using query: %s fields: %s for class: %s in collection: %s",
@@ -2443,8 +2457,9 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		MongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(sourceClass);
 
-		Document mappedFields = getMappedFieldsObject(fields, entity, targetClass);
-		Document mappedQuery = queryMapper.getMappedObject(query, entity);
+		QueryContext queryContext = queryOperations.createQueryContext(new BasicQuery(query, fields));
+		Document mappedFields = queryContext.getMappedFields(entity, targetClass, projectionFactory);
+		Document mappedQuery = queryContext.getMappedQuery(entity);
 
 		if (LOGGER.isDebugEnabled()) {
 			LOGGER.debug("find using query: {} fields: {} for class: {} in collection: {}",
@@ -2675,7 +2690,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	/**
 	 * Prepare the collection before any processing is done using it. This allows a convenient way to apply settings like
-	 * slaveOk() etc. Can be overridden in sub-classes.
+	 * withCodecRegistry() etc. Can be overridden in sub-classes.
 	 *
 	 * @param collection
 	 */
@@ -2834,7 +2849,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 	/**
 	 * Simple {@link ReactiveCollectionCallback} that takes a query {@link Document} plus an optional fields specification
-	 * {@link Document} and executes that against the {@link DBCollection}.
+	 * {@link Document} and executes that against the {@link MongoCollection}.
 	 *
 	 * @author Oliver Gierke
 	 * @author Thomas Risberg
@@ -3288,7 +3303,8 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 
 		@Override
 		public ReadPreference getReadPreference() {
-			return query.getMeta().getFlags().contains(CursorOption.SLAVE_OK) ? ReadPreference.primaryPreferred() : null;
+			return (query.getMeta().getFlags().contains(CursorOption.SECONDARY_READS)
+					|| query.getMeta().getFlags().contains(CursorOption.SLAVE_OK)) ? ReadPreference.primaryPreferred() : null;
 		}
 	}
 
@@ -3340,7 +3356,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		 * @see org.springframework.data.mongodb.core.ReactiveMongoTemplate#getCollection(java.lang.String)
 		 */
 		@Override
-		public MongoCollection<Document> getCollection(String collectionName) {
+		public Mono<MongoCollection<Document>> getCollection(String collectionName) {
 
 			// native MongoDB objects that offer methods with ClientSession must not be proxied.
 			return delegate.getCollection(collectionName);
@@ -3351,7 +3367,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		 * @see org.springframework.data.mongodb.core.ReactiveMongoTemplate#getMongoDatabase()
 		 */
 		@Override
-		public MongoDatabase getMongoDatabase() {
+		public Mono<MongoDatabase> getMongoDatabase() {
 
 			// native MongoDB objects that offer methods with ClientSession must not be proxied.
 			return delegate.getMongoDatabase();

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveRemoveOperation.java

@@ -111,13 +111,13 @@ public interface ReactiveRemoveOperation {
 		/**
 		 * Set the filter {@link CriteriaDefinition criteria} to be used.
 		 *
-		 * @param criteriaDefinition must not be {@literal null}.
+		 * @param criteria must not be {@literal null}.
 		 * @return new instance of {@link TerminatingRemove}.
 		 * @throws IllegalArgumentException if query is {@literal null}.
 		 * @since 3.0
 		 */
-		default TerminatingRemove<T> matching(CriteriaDefinition criteriaDefinition) {
-			return matching(Query.query(criteriaDefinition));
+		default TerminatingRemove<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
 		}
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/ReactiveUpdateOperation.java

@@ -176,13 +176,13 @@ public interface ReactiveUpdateOperation {
 		/**
 		 * Set the filter {@link CriteriaDefinition criteria} to be used.
 		 *
-		 * @param criteriaDefinition must not be {@literal null}.
+		 * @param criteria must not be {@literal null}.
 		 * @return new instance of {@link UpdateWithUpdate}.
 		 * @throws IllegalArgumentException if query is {@literal null}.
 		 * @since 3.0
 		 */
-		default UpdateWithUpdate<T> matching(CriteriaDefinition criteriaDefinition) {
-			return matching(Query.query(criteriaDefinition));
+		default UpdateWithUpdate<T> matching(CriteriaDefinition criteria) {
+			return matching(Query.query(criteria));
 		}
 	}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/SimpleReactiveMongoDatabaseFactory.java

@@ -18,6 +18,7 @@ package org.springframework.data.mongodb.core;
 import lombok.Value;
 import reactor.core.publisher.Mono;
 
+import org.bson.codecs.configuration.CodecRegistry;
 import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.beans.factory.DisposableBean;
 import org.springframework.dao.DataAccessException;
@@ -41,6 +42,7 @@ import com.mongodb.reactivestreams.client.MongoDatabase;
  *
  * @author Mark Paluch
  * @author Christoph Strobl
+ * @author Mathieu Ouellet
  * @since 2.0
  */
 public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, ReactiveMongoDatabaseFactory {
@@ -99,7 +101,7 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.ReactiveMongoDbFactory#getMongoDatabase()
 	 */
-	public MongoDatabase getMongoDatabase() throws DataAccessException {
+	public Mono<MongoDatabase> getMongoDatabase() throws DataAccessException {
 		return getMongoDatabase(databaseName);
 	}
 
@@ -107,12 +109,16 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.ReactiveMongoDbFactory#getMongoDatabase(java.lang.String)
 	 */
-	public MongoDatabase getMongoDatabase(String dbName) throws DataAccessException {
+	public Mono<MongoDatabase> getMongoDatabase(String dbName) throws DataAccessException {
 
 		Assert.hasText(dbName, "Database name must not be empty.");
 
-		MongoDatabase db = mongo.getDatabase(dbName);
-		return writeConcern != null ? db.withWriteConcern(writeConcern) : db;
+		return Mono.fromSupplier(() -> {
+
+			MongoDatabase db = mongo.getDatabase(dbName);
+
+			return writeConcern != null ? db.withWriteConcern(writeConcern) : db;
+		});
 	}
 
 	/**
@@ -135,6 +141,15 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 		return this.exceptionTranslator;
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getCodecRegistry()
+	 */
+	@Override
+	public CodecRegistry getCodecRegistry() {
+		return this.mongo.getDatabase(databaseName).getCodecRegistry();
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.ReactiveMongoDbFactory#getSession(com.mongodb.ClientSessionOptions)
@@ -171,8 +186,8 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getMongoDatabase()
 		 */
 		@Override
-		public MongoDatabase getMongoDatabase() throws DataAccessException {
-			return decorateDatabase(delegate.getMongoDatabase());
+		public Mono<MongoDatabase> getMongoDatabase() throws DataAccessException {
+			return delegate.getMongoDatabase().map(this::decorateDatabase);
 		}
 
 		/*
@@ -180,8 +195,8 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getMongoDatabase(java.lang.String)
 		 */
 		@Override
-		public MongoDatabase getMongoDatabase(String dbName) throws DataAccessException {
-			return decorateDatabase(delegate.getMongoDatabase(dbName));
+		public Mono<MongoDatabase> getMongoDatabase(String dbName) throws DataAccessException {
+			return delegate.getMongoDatabase(dbName).map(this::decorateDatabase);
 		}
 
 		/*
@@ -193,6 +208,15 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 			return delegate.getExceptionTranslator();
 		}
 
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getCodecRegistry()
+		 */
+		@Override
+		public CodecRegistry getCodecRegistry() {
+			return delegate.getCodecRegistry();
+		}
+
 		/*
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.ReactiveMongoDatabaseFactory#getSession(com.mongodb.ClientSessionOptions)
@@ -242,7 +266,7 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 			factory.addAdvice(new SessionAwareMethodInterceptor<>(session, target, ClientSession.class, MongoDatabase.class,
 					this::proxyDatabase, MongoCollection.class, this::proxyCollection));
 
-			return targetType.cast(factory.getProxy());
+			return targetType.cast(factory.getProxy(target.getClass().getClassLoader()));
 		}
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/WriteResultChecking.java

@@ -16,8 +16,8 @@
 package org.springframework.data.mongodb.core;
 
 /**
- * Enum to represent how strict the check of {@link com.mongodb.WriteResult} shall be. It can either be skipped entirely
- * (use {@link #NONE}) or cause an exception to be thrown {@link #EXCEPTION}.
+ * Enum to represent how strict the check of {@link com.mongodb.WriteConcernResult} shall be. It can either be skipped
+ * entirely (use {@link #NONE}) or cause an exception to be thrown {@link #EXCEPTION}.
  *
  * @author Thomas Risberg
  * @author Oliver Gierke

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.java

@@ -17,6 +17,7 @@ package org.springframework.data.mongodb.core.aggregation;
 
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collection;
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.List;
@@ -100,14 +101,14 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 		return value;
 	}
 
-	protected List<Object> append(Object value) {
+	protected List<Object> append(Object value, Expand expandList) {
 
 		if (this.value instanceof List) {
 
 			List<Object> clone = new ArrayList<Object>((List) this.value);
 
-			if (value instanceof List) {
-				clone.addAll((List) value);
+			if (value instanceof Collection && Expand.EXPAND_VALUES.equals(expandList)) {
+				clone.addAll((Collection<?>) value);
 			} else {
 				clone.add(value);
 			}
@@ -117,6 +118,17 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 		return Arrays.asList(this.value, value);
 	}
 
+	/**
+	 * Expand a nested list of values to single entries or keep the list.
+	 */
+	protected enum Expand {
+		EXPAND_VALUES, KEEP_SOURCE
+	}
+
+	protected List<Object> append(Object value) {
+		return append(value, Expand.EXPAND_VALUES);
+	}
+
 	@SuppressWarnings("unchecked")
 	protected java.util.Map<String, Object> append(String key, Object value) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AccumulatorOperators.java

@@ -34,7 +34,7 @@ public class AccumulatorOperators {
 	 * Take the numeric value referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link AccumulatorOperatorFactory}.
 	 */
 	public static AccumulatorOperatorFactory valueOf(String fieldReference) {
 		return new AccumulatorOperatorFactory(fieldReference);
@@ -44,7 +44,7 @@ public class AccumulatorOperators {
 	 * Take the numeric value referenced resulting from given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link AccumulatorOperatorFactory}.
 	 */
 	public static AccumulatorOperatorFactory valueOf(AggregationExpression expression) {
 		return new AccumulatorOperatorFactory(expression);
@@ -86,7 +86,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and calculates and
 		 * returns the sum.
 		 *
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public Sum sum() {
 			return usesFieldRef() ? Sum.sumOf(fieldReference) : Sum.sumOf(expression);
@@ -96,7 +96,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and returns the
 		 * average value.
 		 *
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public Avg avg() {
 			return usesFieldRef() ? Avg.avgOf(fieldReference) : Avg.avgOf(expression);
@@ -106,7 +106,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and returns the
 		 * maximum value.
 		 *
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public Max max() {
 			return usesFieldRef() ? Max.maxOf(fieldReference) : Max.maxOf(expression);
@@ -116,7 +116,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and returns the
 		 * minimum value.
 		 *
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public Min min() {
 			return usesFieldRef() ? Min.minOf(fieldReference) : Min.minOf(expression);
@@ -126,7 +126,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and calculates the
 		 * population standard deviation of the input values.
 		 *
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public StdDevPop stdDevPop() {
 			return usesFieldRef() ? StdDevPop.stdDevPopOf(fieldReference) : StdDevPop.stdDevPopOf(expression);
@@ -136,7 +136,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated numeric value expression and calculates the
 		 * sample standard deviation of the input values.
 		 *
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public StdDevSamp stdDevSamp() {
 			return usesFieldRef() ? StdDevSamp.stdDevSampOf(fieldReference) : StdDevSamp.stdDevSampOf(expression);
@@ -167,7 +167,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Sum}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public static Sum sumOf(String fieldReference) {
 
@@ -179,7 +179,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Sum}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public static Sum sumOf(AggregationExpression expression) {
 
@@ -192,7 +192,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public Sum and(String fieldReference) {
 
@@ -205,7 +205,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public Sum and(AggregationExpression expression) {
 
@@ -264,7 +264,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Avg}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public static Avg avgOf(String fieldReference) {
 
@@ -276,7 +276,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Avg}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public static Avg avgOf(AggregationExpression expression) {
 
@@ -289,7 +289,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public Avg and(String fieldReference) {
 
@@ -302,7 +302,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public Avg and(AggregationExpression expression) {
 
@@ -347,7 +347,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Max}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public static Max maxOf(String fieldReference) {
 
@@ -359,7 +359,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Max}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public static Max maxOf(AggregationExpression expression) {
 
@@ -372,7 +372,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public Max and(String fieldReference) {
 
@@ -385,7 +385,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public Max and(AggregationExpression expression) {
 
@@ -430,7 +430,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Min}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public static Min minOf(String fieldReference) {
 
@@ -442,7 +442,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link Min}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public static Min minOf(AggregationExpression expression) {
 
@@ -455,7 +455,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public Min and(String fieldReference) {
 
@@ -468,7 +468,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public Min and(AggregationExpression expression) {
 
@@ -513,7 +513,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link StdDevPop}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public static StdDevPop stdDevPopOf(String fieldReference) {
 
@@ -525,7 +525,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link StdDevPop} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public static StdDevPop stdDevPopOf(AggregationExpression expression) {
 
@@ -538,7 +538,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public StdDevPop and(String fieldReference) {
 
@@ -551,7 +551,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public StdDevPop and(AggregationExpression expression) {
 
@@ -596,7 +596,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link StdDevSamp}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public static StdDevSamp stdDevSampOf(String fieldReference) {
 
@@ -608,7 +608,7 @@ public class AccumulatorOperators {
 		 * Creates new {@link StdDevSamp}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public static StdDevSamp stdDevSampOf(AggregationExpression expression) {
 
@@ -621,7 +621,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public StdDevSamp and(String fieldReference) {
 
@@ -634,7 +634,7 @@ public class AccumulatorOperators {
 		 * <strong>NOTE:</strong> Only possible in {@code $project} stage.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public StdDevSamp and(AggregationExpression expression) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AddFieldsOperation.java

@@ -99,6 +99,10 @@ public class AddFieldsOperation extends DocumentEnhancingOperation {
 		return new AddFieldsOperationBuilder(getValueMap());
 	}
 
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.DocumentEnhancingOperation#mongoOperator()
+	 */
 	@Override
 	protected String mongoOperator() {
 		return "$addFields";

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/Aggregation.java

@@ -96,7 +96,7 @@ public class Aggregation {
 	public static final AggregationOperationContext DEFAULT_CONTEXT = AggregationOperationRenderer.DEFAULT_CONTEXT;
 	public static final AggregationOptions DEFAULT_OPTIONS = newAggregationOptions().build();
 
-	protected final List<AggregationOperation> operations;
+	protected final AggregationPipeline pipeline;
 	private final AggregationOptions options;
 
 	/**
@@ -133,13 +133,13 @@ public class Aggregation {
 	 * supported in MongoDB version 2.6+.
 	 *
 	 * @param options must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Aggregation}.
 	 * @since 1.6
 	 */
 	public Aggregation withOptions(AggregationOptions options) {
 
 		Assert.notNull(options, "AggregationOptions must not be null.");
-		return new Aggregation(this.operations, options);
+		return new Aggregation(this.pipeline.getOperations(), options);
 	}
 
 	/**
@@ -202,26 +202,10 @@ public class Aggregation {
 		Assert.notNull(aggregationOperations, "AggregationOperations must not be null!");
 		Assert.notNull(options, "AggregationOptions must not be null!");
 
-		// check $out/$merge is the last operation if it exists
-		for (AggregationOperation aggregationOperation : aggregationOperations) {
-
-			if (aggregationOperation instanceof OutOperation && !isLast(aggregationOperation, aggregationOperations)) {
-				throw new IllegalArgumentException("The $out operator must be the last stage in the pipeline.");
-			}
-
-			if (aggregationOperation instanceof MergeOperation && !isLast(aggregationOperation, aggregationOperations)) {
-				throw new IllegalArgumentException("The $merge operator must be the last stage in the pipeline.");
-			}
-		}
-
-		this.operations = aggregationOperations;
+		this.pipeline = new AggregationPipeline(aggregationOperations);
 		this.options = options;
 	}
 
-	private boolean isLast(AggregationOperation aggregationOperation, List<AggregationOperation> aggregationOperations) {
-		return aggregationOperations.indexOf(aggregationOperation) == aggregationOperations.size() - 1;
-	}
-
 	/**
 	 * Get the {@link AggregationOptions}.
 	 *
@@ -259,7 +243,7 @@ public class Aggregation {
 	 * Creates a new {@link ProjectionOperation} including the given fields.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ProjectionOperation}.
 	 */
 	public static ProjectionOperation project(String... fields) {
 		return project(fields(fields));
@@ -269,7 +253,7 @@ public class Aggregation {
 	 * Creates a new {@link ProjectionOperation} including the given {@link Fields}.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ProjectionOperation}.
 	 */
 	public static ProjectionOperation project(Fields fields) {
 		return new ProjectionOperation(fields);
@@ -292,7 +276,7 @@ public class Aggregation {
 	 * Factory method to create a new {@link UnwindOperation} for the field with the given name.
 	 *
 	 * @param field must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link UnwindOperation}.
 	 */
 	public static UnwindOperation unwind(String field) {
 		return new UnwindOperation(field(field));
@@ -302,7 +286,7 @@ public class Aggregation {
 	 * Factory method to create a new {@link ReplaceRootOperation} for the field with the given name.
 	 *
 	 * @param fieldName must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link ReplaceRootOperation}.
 	 * @since 1.10
 	 */
 	public static ReplaceRootOperation replaceRoot(String fieldName) {
@@ -314,7 +298,7 @@ public class Aggregation {
 	 * {@link AggregationExpression}.
 	 *
 	 * @param aggregationExpression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ReplaceRootOperation}.
 	 * @since 1.10
 	 */
 	public static ReplaceRootOperation replaceRoot(AggregationExpression aggregationExpression) {
@@ -380,7 +364,7 @@ public class Aggregation {
 	 * Creates a new {@link GroupOperation} for the given fields.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link GroupOperation}.
 	 */
 	public static GroupOperation group(String... fields) {
 		return group(fields(fields));
@@ -401,7 +385,7 @@ public class Aggregation {
 	 * {@link GraphLookupOperation} given {@literal fromCollection}.
 	 *
 	 * @param fromCollection must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link StartWithBuilder} for creating a {@link GraphLookupOperation}.
 	 * @since 1.10
 	 */
 	public static StartWithBuilder graphLookup(String fromCollection) {
@@ -412,7 +396,7 @@ public class Aggregation {
 	 * Factory method to create a new {@link SortOperation} for the given {@link Sort}.
 	 *
 	 * @param sort must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SortOperation}.
 	 */
 	public static SortOperation sort(Sort sort) {
 		return new SortOperation(sort);
@@ -423,7 +407,7 @@ public class Aggregation {
 	 *
 	 * @param direction must not be {@literal null}.
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SortOperation}.
 	 */
 	public static SortOperation sort(Direction direction, String... fields) {
 		return new SortOperation(Sort.by(direction, fields));
@@ -433,7 +417,7 @@ public class Aggregation {
 	 * Creates a new {@link SortByCountOperation} given {@literal groupByField}.
 	 *
 	 * @param field must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link SortByCountOperation}.
 	 * @since 2.1
 	 */
 	public static SortByCountOperation sortByCount(String field) {
@@ -444,7 +428,7 @@ public class Aggregation {
 	 * Creates a new {@link SortByCountOperation} given {@link AggregationExpression group and sort expression}.
 	 *
 	 * @param groupAndSortExpression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SortByCountOperation}.
 	 * @since 2.1
 	 */
 	public static SortByCountOperation sortByCount(AggregationExpression groupAndSortExpression) {
@@ -455,9 +439,10 @@ public class Aggregation {
 	 * Creates a new {@link SkipOperation} skipping the given number of elements.
 	 *
 	 * @param elementsToSkip must not be less than zero.
-	 * @return
+	 * @return new instance of {@link SkipOperation}.
 	 * @deprecated prepare to get this one removed in favor of {@link #skip(long)}.
 	 */
+	@Deprecated
 	public static SkipOperation skip(int elementsToSkip) {
 		return new SkipOperation(elementsToSkip);
 	}
@@ -466,7 +451,7 @@ public class Aggregation {
 	 * Creates a new {@link SkipOperation} skipping the given number of elements.
 	 *
 	 * @param elementsToSkip must not be less than zero.
-	 * @return
+	 * @return new instance of {@link SkipOperation}.
 	 */
 	public static SkipOperation skip(long elementsToSkip) {
 		return new SkipOperation(elementsToSkip);
@@ -476,7 +461,7 @@ public class Aggregation {
 	 * Creates a new {@link LimitOperation} limiting the result to the given number of elements.
 	 *
 	 * @param maxElements must not be less than zero.
-	 * @return
+	 * @return new instance of {@link LimitOperation}.
 	 */
 	public static LimitOperation limit(long maxElements) {
 		return new LimitOperation(maxElements);
@@ -486,7 +471,7 @@ public class Aggregation {
 	 * Creates a new {@link SampleOperation} to select the specified number of documents from its input randomly.
 	 *
 	 * @param sampleSize must not be less than zero.
-	 * @return
+	 * @return new instance of {@link SampleOperation}.
 	 * @since 2.0
 	 */
 	public static SampleOperation sample(long sampleSize) {
@@ -497,7 +482,7 @@ public class Aggregation {
 	 * Creates a new {@link MatchOperation} using the given {@link Criteria}.
 	 *
 	 * @param criteria must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link MatchOperation}.
 	 */
 	public static MatchOperation match(Criteria criteria) {
 		return new MatchOperation(criteria);
@@ -507,7 +492,7 @@ public class Aggregation {
 	 * Creates a new {@link MatchOperation} using the given {@link CriteriaDefinition}.
 	 *
 	 * @param criteria must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link MatchOperation}.
 	 * @since 1.10
 	 */
 	public static MatchOperation match(CriteriaDefinition criteria) {
@@ -520,7 +505,7 @@ public class Aggregation {
 	 *
 	 * @param query must not be {@literal null}.
 	 * @param distanceField must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link GeoNearOperation}.
 	 * @since 1.7
 	 */
 	public static GeoNearOperation geoNear(NearQuery query, String distanceField) {
@@ -546,7 +531,7 @@ public class Aggregation {
 	 *          collection in the current database if one does not already exist. The collection is not visible until the
 	 *          aggregation completes. If the aggregation fails, MongoDB does not create the collection. Must not be
 	 *          {@literal null}.
-	 * @return
+	 * @return new instance of {@link OutOperation}.
 	 */
 	public static OutOperation out(String outCollectionName) {
 		return new OutOperation(outCollectionName);
@@ -556,7 +541,7 @@ public class Aggregation {
 	 * Creates a new {@link BucketOperation} given {@literal groupByField}.
 	 *
 	 * @param groupByField must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link BucketOperation}.
 	 * @since 1.10
 	 */
 	public static BucketOperation bucket(String groupByField) {
@@ -567,7 +552,7 @@ public class Aggregation {
 	 * Creates a new {@link BucketOperation} given {@link AggregationExpression group-by expression}.
 	 *
 	 * @param groupByExpression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BucketOperation}.
 	 * @since 1.10
 	 */
 	public static BucketOperation bucket(AggregationExpression groupByExpression) {
@@ -579,7 +564,7 @@ public class Aggregation {
 	 *
 	 * @param groupByField must not be {@literal null} or empty.
 	 * @param buckets number of buckets, must be a positive integer.
-	 * @return
+	 * @return new instance of {@link BucketAutoOperation}.
 	 * @since 1.10
 	 */
 	public static BucketAutoOperation bucketAuto(String groupByField, int buckets) {
@@ -591,7 +576,7 @@ public class Aggregation {
 	 *
 	 * @param groupByExpression must not be {@literal null}.
 	 * @param buckets number of buckets, must be a positive integer.
-	 * @return
+	 * @return new instance of {@link BucketAutoOperation}.
 	 * @since 1.10
 	 */
 	public static BucketAutoOperation bucketAuto(AggregationExpression groupByExpression, int buckets) {
@@ -601,7 +586,7 @@ public class Aggregation {
 	/**
 	 * Creates a new {@link FacetOperation}.
 	 *
-	 * @return
+	 * @return new instance of {@link FacetOperation}.
 	 * @since 1.10
 	 */
 	public static FacetOperation facet() {
@@ -612,7 +597,7 @@ public class Aggregation {
 	 * Creates a new {@link FacetOperationBuilder} given {@link Aggregation}.
 	 *
 	 * @param aggregationOperations the sub-pipeline, must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link FacetOperation}.
 	 * @since 1.10
 	 */
 	public static FacetOperationBuilder facet(AggregationOperation... aggregationOperations) {
@@ -660,9 +645,9 @@ public class Aggregation {
 	/**
 	 * Creates a new {@link RedactOperation} that can restrict the content of a document based on information stored
 	 * within the document itself.
-	 * 
+	 *
 	 * <pre class="code">
-	 * 
+	 *
 	 * Aggregation.redact(ConditionalOperators.when(Criteria.where("level").is(5)) //
 	 * 		.then(RedactOperation.PRUNE) //
 	 * 		.otherwise(RedactOperation.DESCEND));
@@ -681,7 +666,7 @@ public class Aggregation {
 	 * Creates a new {@link Fields} instance for the given field names.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Fields}.
 	 * @see Fields#fields(String...)
 	 */
 	public static Fields fields(String... fields) {
@@ -693,7 +678,7 @@ public class Aggregation {
 	 *
 	 * @param name must not be {@literal null} or empty.
 	 * @param target must not be {@literal null} or empty.
-	 * @return
+	 * @return new instance of {@link Fields}.
 	 */
 	public static Fields bind(String name, String target) {
 		return Fields.from(field(name, target));
@@ -702,7 +687,7 @@ public class Aggregation {
 	/**
 	 * Returns a new {@link AggregationOptions.Builder}.
 	 *
-	 * @return
+	 * @return new instance of {@link AggregationOptions.Builder}.
 	 * @since 1.6
 	 */
 	public static AggregationOptions.Builder newAggregationOptions() {
@@ -717,7 +702,15 @@ public class Aggregation {
 	 * @since 2.1
 	 */
 	public List<Document> toPipeline(AggregationOperationContext rootContext) {
-		return AggregationOperationRenderer.toDocument(operations, rootContext);
+		return pipeline.toDocuments(rootContext);
+	}
+
+	/**
+	 * @return the {@link AggregationPipeline}.
+	 * @since 3.0.2
+	 */
+	public AggregationPipeline getPipeline() {
+		return pipeline;
 	}
 
 	/**
@@ -764,7 +757,7 @@ public class Aggregation {
 		 * otherwise.
 		 *
 		 * @param fieldRef may be {@literal null}.
-		 * @return
+		 * @return {@literal true} if the given field refers to a {@link SystemVariable}.
 		 */
 		public static boolean isReferingToSystemVariable(@Nullable String fieldRef) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationExpression.java

@@ -31,8 +31,8 @@ public interface AggregationExpression {
 	 * Turns the {@link AggregationExpression} into a {@link Document} within the given
 	 * {@link AggregationOperationContext}.
 	 *
-	 * @param context
-	 * @return
+	 * @param context must not be {@literal null}.
+	 * @return the MongoDB native ({@link Document}) form of the expression.
 	 */
 	Document toDocument(AggregationOperationContext context);
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationExpressionTransformer.java

@@ -53,7 +53,7 @@ interface AggregationExpressionTransformer
 		 * @param currentNode must not be {@literal null}.
 		 * @param parentNode may be {@literal null}.
 		 * @param previousOperationObject may be {@literal null}.
-		 * @param aggregationContext must not be {@literal null}.
+		 * @param context must not be {@literal null}.
 		 */
 		public AggregationExpressionTransformationContext(T currentNode, @Nullable ExpressionNode parentNode,
 				@Nullable Document previousOperationObject, AggregationOperationContext context) {
@@ -67,7 +67,7 @@ interface AggregationExpressionTransformer
 		/**
 		 * Returns the underlying {@link AggregationOperationContext}.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public AggregationOperationContext getAggregationContext() {
 			return aggregationContext;
@@ -76,7 +76,7 @@ interface AggregationExpressionTransformer
 		/**
 		 * Returns the {@link FieldReference} for the current {@link ExpressionNode}.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public FieldReference getFieldReference() {
 			return aggregationContext.getReference(getCurrentNode().getName());

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationFunctionExpressions.java

@@ -41,7 +41,7 @@ public enum AggregationFunctionExpressions {
 	 * Returns an {@link AggregationExpression} build from the current {@link Enum} name and the given parameters.
 	 *
 	 * @param parameters must not be {@literal null}
-	 * @return
+	 * @return new instance of {@link AggregationExpression}.
 	 */
 	public AggregationExpression of(Object... parameters) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationOperation.java

@@ -54,4 +54,15 @@ public interface AggregationOperation {
 	default List<Document> toPipelineStages(AggregationOperationContext context) {
 		return Collections.singletonList(toDocument(context));
 	}
+
+	/**
+	 * Return the MongoDB operator that is used for this {@link AggregationOperation}. Aggregation operations should
+	 * implement this method to avoid document rendering.
+	 *
+	 * @return the operator used for this {@link AggregationOperation}.
+	 * @since 3.0.2
+	 */
+	default String getOperator() {
+		return toDocument(Aggregation.DEFAULT_CONTEXT).keySet().iterator().next();
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationOperationContext.java

@@ -57,20 +57,20 @@ public interface AggregationOperationContext {
 	Document getMappedObject(Document document, @Nullable Class<?> type);
 
 	/**
-	 * Returns a {@link FieldReference} for the given field or {@literal null} if the context does not expose the given
-	 * field.
+	 * Returns a {@link FieldReference} for the given field.
 	 *
 	 * @param field must not be {@literal null}.
-	 * @return
+	 * @return the {@link FieldReference} for the given {@link Field}.
+	 * @throws IllegalArgumentException if the context does not expose a field with the given name
 	 */
 	FieldReference getReference(Field field);
 
 	/**
-	 * Returns the {@link FieldReference} for the field with the given name or {@literal null} if the context does not
-	 * expose a field with the given name.
+	 * Returns the {@link FieldReference} for the field with the given name.
 	 *
 	 * @param name must not be {@literal null} or empty.
-	 * @return
+	 * @return the {@link FieldReference} for the field with given {@literal name}.
+	 * @throws IllegalArgumentException if the context does not expose a field with the given name
 	 */
 	FieldReference getReference(String name);
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationOptions.java

@@ -52,6 +52,7 @@ public class AggregationOptions {
 	private final Optional<Collation> collation;
 	private final Optional<String> comment;
 	private Duration maxTime = Duration.ZERO;
+	private ResultOptions resultOptions = ResultOptions.READ;
 
 	/**
 	 * Creates a new {@link AggregationOptions}.
@@ -70,7 +71,7 @@ public class AggregationOptions {
 	 * @param allowDiskUse whether to off-load intensive sort-operations to disk.
 	 * @param explain whether to get the execution plan for the aggregation instead of the actual results.
 	 * @param cursor can be {@literal null}, used to pass additional options (such as {@code batchSize}) to the
-	 *          aggregation.
+	 * aggregation.
 	 * @param collation collation for string comparison. Can be {@literal null}.
 	 * @since 2.0
 	 */
@@ -85,7 +86,7 @@ public class AggregationOptions {
 	 * @param allowDiskUse whether to off-load intensive sort-operations to disk.
 	 * @param explain whether to get the execution plan for the aggregation instead of the actual results.
 	 * @param cursor can be {@literal null}, used to pass additional options (such as {@code batchSize}) to the
-	 *          aggregation.
+	 * aggregation.
 	 * @param collation collation for string comparison. Can be {@literal null}.
 	 * @param comment execution comment. Can be {@literal null}.
 	 * @since 2.2
@@ -151,7 +152,7 @@ public class AggregationOptions {
 	 * Enables writing to temporary files. When set to true, aggregation stages can write data to the _tmp subdirectory in
 	 * the dbPath directory.
 	 *
-	 * @return
+	 * @return {@literal true} if enabled.
 	 */
 	public boolean isAllowDiskUse() {
 		return allowDiskUse;
@@ -160,7 +161,7 @@ public class AggregationOptions {
 	/**
 	 * Specifies to return the information on the processing of the pipeline.
 	 *
-	 * @return
+	 * @return {@literal true} if enabled.
 	 */
 	public boolean isExplain() {
 		return explain;
@@ -185,7 +186,7 @@ public class AggregationOptions {
 	/**
 	 * Specify a document that contains options that control the creation of the cursor object.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public Optional<Document> getCursor() {
 		return cursor;
@@ -194,7 +195,7 @@ public class AggregationOptions {
 	/**
 	 * Get collation settings for string comparison.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.0
 	 */
 	public Optional<Collation> getCollation() {
@@ -204,7 +205,7 @@ public class AggregationOptions {
 	/**
 	 * Get the comment for the aggregation.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.2
 	 */
 	public Optional<String> getComment() {
@@ -219,6 +220,15 @@ public class AggregationOptions {
 		return maxTime;
 	}
 
+	/**
+	 * @return {@literal true} to skip results when running an aggregation. Useful in combination with {@code $merge} or
+	 *         {@code $out}.
+	 * @since 3.0.2
+	 */
+	public boolean isSkipResults() {
+		return ResultOptions.SKIP.equals(resultOptions);
+	}
+
 	/**
 	 * Returns a new potentially adjusted copy for the given {@code aggregationCommandObject} with the configuration
 	 * applied.
@@ -256,7 +266,7 @@ public class AggregationOptions {
 	/**
 	 * Returns a {@link Document} representation of this {@link AggregationOptions}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public Document toDocument() {
 
@@ -276,7 +286,7 @@ public class AggregationOptions {
 	}
 
 	/**
-	 * @return
+	 * @return {@literal true} if {@link #maxTime} is set to a positive value.
 	 * @since 3.0
 	 */
 	public boolean hasExecutionTimeLimit() {
@@ -309,12 +319,13 @@ public class AggregationOptions {
 		private @Nullable Collation collation;
 		private @Nullable String comment;
 		private @Nullable Duration maxTime;
+		private @Nullable ResultOptions resultOptions;
 
 		/**
 		 * Defines whether to off-load intensive sort-operations to disk.
 		 *
-		 * @param allowDiskUse
-		 * @return
+		 * @param allowDiskUse use {@literal true} to allow disk use during the aggregation.
+		 * @return this.
 		 */
 		public Builder allowDiskUse(boolean allowDiskUse) {
 
@@ -325,8 +336,8 @@ public class AggregationOptions {
 		/**
 		 * Defines whether to get the execution plan for the aggregation instead of the actual results.
 		 *
-		 * @param explain
-		 * @return
+		 * @param explain use {@literal true} to enable explain feature.
+		 * @return this.
 		 */
 		public Builder explain(boolean explain) {
 
@@ -337,8 +348,8 @@ public class AggregationOptions {
 		/**
 		 * Additional options to the aggregation.
 		 *
-		 * @param cursor
-		 * @return
+		 * @param cursor must not be {@literal null}.
+		 * @return this.
 		 */
 		public Builder cursor(Document cursor) {
 
@@ -349,8 +360,8 @@ public class AggregationOptions {
 		/**
 		 * Define the initial cursor batch size.
 		 *
-		 * @param batchSize
-		 * @return
+		 * @param batchSize use a positive int.
+		 * @return this.
 		 * @since 2.0
 		 */
 		public Builder cursorBatchSize(int batchSize) {
@@ -363,7 +374,7 @@ public class AggregationOptions {
 		 * Define collation settings for string comparison.
 		 *
 		 * @param collation can be {@literal null}.
-		 * @return
+		 * @return this.
 		 * @since 2.0
 		 */
 		public Builder collation(@Nullable Collation collation) {
@@ -376,7 +387,7 @@ public class AggregationOptions {
 		 * Define a comment to describe the execution.
 		 *
 		 * @param comment can be {@literal null}.
-		 * @return
+		 * @return this.
 		 * @since 2.2
 		 */
 		public Builder comment(@Nullable String comment) {
@@ -389,7 +400,7 @@ public class AggregationOptions {
 		 * Set the time limit for processing.
 		 *
 		 * @param maxTime {@link Duration#ZERO} is used for the default unbounded behavior. {@link Duration#isNegative()
-		 *          Negative} values will be ignored.
+		 * Negative} values will be ignored.
 		 * @return this.
 		 * @since 3.0
 		 */
@@ -399,10 +410,24 @@ public class AggregationOptions {
 			return this;
 		}
 
+		/**
+		 * Run the aggregation, but do NOT read the aggregation result from the store. <br />
+		 * If the expected result of the aggregation is rather large, eg. when using an {@literal $out} operation, this
+		 * option allows to execute the aggregation without having the cursor return the operation result.
+		 *
+		 * @return this.
+		 * @since 3.0.2
+		 */
+		public Builder skipOutput() {
+
+			this.resultOptions = ResultOptions.SKIP;
+			return this;
+		}
+
 		/**
 		 * Returns a new {@link AggregationOptions} instance with the given configuration.
 		 *
-		 * @return
+		 * @return new instance of {@link AggregationOptions}.
 		 */
 		public AggregationOptions build() {
 
@@ -410,8 +435,26 @@ public class AggregationOptions {
 			if (maxTime != null) {
 				options.maxTime = maxTime;
 			}
+			if (resultOptions != null) {
+				options.resultOptions = resultOptions;
+			}
 
 			return options;
 		}
 	}
+
+	/**
+	 * @since 3.0
+	 */
+	private enum ResultOptions {
+
+		/**
+		 * Just do it!, and do not read the operation result.
+		 */
+		SKIP,
+		/**
+		 * Read the aggregation result from the cursor.
+		 */
+		READ;
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationPipeline.java

@@ -0,0 +1,111 @@
+/*
+ * Copyright 2020 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.data.mongodb.core.aggregation;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+
+import org.bson.Document;
+import org.springframework.util.Assert;
+
+/**
+ * The {@link AggregationPipeline} holds the collection of {@link AggregationOperation aggregation stages}.
+ *
+ * @author Christoph Strobl
+ * @since 3.0.2
+ */
+public class AggregationPipeline {
+
+	private final List<AggregationOperation> pipeline;
+
+	/**
+	 * Create an empty pipeline
+	 */
+	public AggregationPipeline() {
+		this(new ArrayList<>());
+	}
+
+	/**
+	 * Create a new pipeline with given {@link AggregationOperation stages}.
+	 *
+	 * @param aggregationOperations must not be {@literal null}.
+	 */
+	public AggregationPipeline(List<AggregationOperation> aggregationOperations) {
+		pipeline = new ArrayList<>(aggregationOperations);
+	}
+
+	/**
+	 * Append the given {@link AggregationOperation stage} to the pipeline.
+	 *
+	 * @param aggregationOperation must not be {@literal null}.
+	 * @return this.
+	 */
+	public AggregationPipeline add(AggregationOperation aggregationOperation) {
+
+		Assert.notNull(aggregationOperation, "AggregationOperation must not be null!");
+
+		pipeline.add(aggregationOperation);
+		return this;
+	}
+
+	/**
+	 * Get the list of {@link AggregationOperation aggregation stages}.
+	 *
+	 * @return never {@literal null}.
+	 */
+	public List<AggregationOperation> getOperations() {
+		return Collections.unmodifiableList(pipeline);
+	}
+
+	List<Document> toDocuments(AggregationOperationContext context) {
+
+		verify();
+		return AggregationOperationRenderer.toDocument(pipeline, context);
+	}
+
+	/**
+	 * @return {@literal true} if the last aggregation stage is either {@literal $out} or {@literal $merge}.
+	 */
+	public boolean isOutOrMerge() {
+
+		if (pipeline.isEmpty()) {
+			return false;
+		}
+
+		String operator = pipeline.get(pipeline.size() - 1).getOperator();
+		return operator.equals("$out") || operator.equals("$merge");
+	}
+
+	void verify() {
+
+		// check $out/$merge is the last operation if it exists
+		for (AggregationOperation aggregationOperation : pipeline) {
+
+			if (aggregationOperation instanceof OutOperation && !isLast(aggregationOperation)) {
+				throw new IllegalArgumentException("The $out operator must be the last stage in the pipeline.");
+			}
+
+			if (aggregationOperation instanceof MergeOperation && !isLast(aggregationOperation)) {
+				throw new IllegalArgumentException("The $merge operator must be the last stage in the pipeline.");
+			}
+		}
+	}
+
+	private boolean isLast(AggregationOperation aggregationOperation) {
+		return pipeline.indexOf(aggregationOperation) == pipeline.size() - 1;
+	}
+}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationResults.java

@@ -59,7 +59,7 @@ public class AggregationResults<T> implements Iterable<T> {
 	/**
 	 * Returns the aggregation results.
 	 *
-	 * @return
+	 * @return the list of already mapped results or an empty one if none found.
 	 */
 	public List<T> getMappedResults() {
 		return mappedResults;
@@ -68,7 +68,7 @@ public class AggregationResults<T> implements Iterable<T> {
 	/**
 	 * Returns the unique mapped result. Assumes no result or exactly one.
 	 *
-	 * @return
+	 * @return the single already mapped result object or raise an error if more than one found.
 	 * @throws IllegalArgumentException in case more than one result is available.
 	 */
 	@Nullable
@@ -88,7 +88,7 @@ public class AggregationResults<T> implements Iterable<T> {
 	/**
 	 * Returns the server that has been used to perform the aggregation.
 	 *
-	 * @return
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	public String getServerUsed() {
@@ -98,7 +98,7 @@ public class AggregationResults<T> implements Iterable<T> {
 	/**
 	 * Returns the raw result that was returned by the server.
 	 *
-	 * @return
+	 * @return the raw response.
 	 * @since 1.6
 	 */
 	public Document getRawResults() {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationSpELExpression.java

@@ -56,7 +56,7 @@ public class AggregationSpELExpression implements AggregationExpression {
 	 *
 	 * @param expressionString must not be {@literal null}.
 	 * @param parameters can be empty.
-	 * @return
+	 * @return new instance of {@link AggregationSpELExpression}.
 	 */
 	public static AggregationSpELExpression expressionOf(String expressionString, Object... parameters) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationUpdate.java

@@ -139,7 +139,7 @@ public class AggregationUpdate extends Aggregation implements UpdateDefinition {
 		setOperation.getFields().forEach(it -> {
 			keysTouched.add(it.getName());
 		});
-		operations.add(setOperation);
+		pipeline.add(setOperation);
 		return this;
 	}
 
@@ -155,7 +155,7 @@ public class AggregationUpdate extends Aggregation implements UpdateDefinition {
 
 		Assert.notNull(unsetOperation, "UnsetOperation must not be null!");
 
-		operations.add(unsetOperation);
+		pipeline.add(unsetOperation);
 		keysTouched.addAll(unsetOperation.removedFieldNames());
 		return this;
 	}
@@ -164,7 +164,7 @@ public class AggregationUpdate extends Aggregation implements UpdateDefinition {
 	 * {@code $replaceWith} replaces the input document with the specified document. The operation replaces all existing
 	 * fields in the input document, including the <strong>_id</strong> field.
 	 *
-	 * @param replaceWithOperation
+	 * @param replaceWithOperation must not be {@literal null}.
 	 * @return this.
 	 * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/replaceWith/">$replaceWith Aggregation
 	 *      Reference</a>
@@ -172,7 +172,7 @@ public class AggregationUpdate extends Aggregation implements UpdateDefinition {
 	public AggregationUpdate replaceWith(ReplaceWithOperation replaceWithOperation) {
 
 		Assert.notNull(replaceWithOperation, "ReplaceWithOperation must not be null!");
-		operations.add(replaceWithOperation);
+		pipeline.add(replaceWithOperation);
 		return this;
 	}
 
@@ -219,8 +219,8 @@ public class AggregationUpdate extends Aggregation implements UpdateDefinition {
 	/**
 	 * Short for {@link #unset(UnsetOperation)}.
 	 *
-	 * @param keys
-	 * @return
+	 * @param keys the fields to remove.
+	 * @return this.
 	 */
 	public AggregationUpdate unset(String... keys) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AggregationUtils.java

@@ -32,9 +32,9 @@ interface AggregationUtils {
 	 * Converts the given {@link Range} into an array of values.
 	 *
 	 * @param range must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
-	public static List<Long> toRangeValues(Range<Long> range) {
+	static List<Long> toRangeValues(Range<Long> range) {
 
 		Assert.notNull(range, "Range must not be null!");
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ArithmeticOperators.java

@@ -38,7 +38,7 @@ public class ArithmeticOperators {
 	 * Take the field referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ArithmeticOperatorFactory}.
 	 */
 	public static ArithmeticOperatorFactory valueOf(String fieldReference) {
 		return new ArithmeticOperatorFactory(fieldReference);
@@ -48,7 +48,7 @@ public class ArithmeticOperators {
 	 * Take the value resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ArithmeticOperatorFactory}.
 	 */
 	public static ArithmeticOperatorFactory valueOf(AggregationExpression expression) {
 		return new ArithmeticOperatorFactory(expression);
@@ -89,7 +89,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the absolute value of the associated number.
 		 *
-		 * @return
+		 * @return new instance of {@link Abs}.
 		 */
 		public Abs abs() {
 			return usesFieldRef() ? Abs.absoluteValueOf(fieldReference) : Abs.absoluteValueOf(expression);
@@ -100,7 +100,7 @@ public class ArithmeticOperators {
 		 * number.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Add}.
 		 */
 		public Add add(String fieldReference) {
 
@@ -113,7 +113,7 @@ public class ArithmeticOperators {
 		 * {@link AggregationExpression} to the associated number.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Add}.
 		 */
 		public Add add(AggregationExpression expression) {
 
@@ -125,7 +125,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that adds the given {@literal value} to the associated number.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Add}.
 		 */
 		public Add add(Number value) {
 
@@ -141,7 +141,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that returns the smallest integer greater than or equal to the
 		 * assoicated number.
 		 *
-		 * @return
+		 * @return new instance of {@link Ceil}.
 		 */
 		public Ceil ceil() {
 			return usesFieldRef() ? Ceil.ceilValueOf(fieldReference) : Ceil.ceilValueOf(expression);
@@ -152,7 +152,7 @@ public class ArithmeticOperators {
 		 * {@literal fieldReference}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Divide}.
 		 */
 		public Divide divideBy(String fieldReference) {
 
@@ -165,7 +165,7 @@ public class ArithmeticOperators {
 		 * {@literal expression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Divide}.
 		 */
 		public Divide divideBy(AggregationExpression expression) {
 
@@ -176,8 +176,8 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that divides the associated number by given {@literal value}.
 		 *
-		 * @param value
-		 * @return
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Divide}.
 		 */
 		public Divide divideBy(Number value) {
 
@@ -192,7 +192,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that raises Euler’s number (i.e. e ) on the associated number.
 		 *
-		 * @return
+		 * @return new instance of {@link Exp}.
 		 */
 		public Exp exp() {
 			return usesFieldRef() ? Exp.expValueOf(fieldReference) : Exp.expValueOf(expression);
@@ -202,7 +202,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that returns the largest integer less than or equal to the associated
 		 * number.
 		 *
-		 * @return
+		 * @return new instance of {@link Floor}.
 		 */
 		public Floor floor() {
 			return usesFieldRef() ? Floor.floorValueOf(fieldReference) : Floor.floorValueOf(expression);
@@ -212,7 +212,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that calculates the natural logarithm ln (i.e loge) of the assoicated
 		 * number.
 		 *
-		 * @return
+		 * @return new instance of {@link Ln}.
 		 */
 		public Ln ln() {
 			return usesFieldRef() ? Ln.lnValueOf(fieldReference) : Ln.lnValueOf(expression);
@@ -223,7 +223,7 @@ public class ArithmeticOperators {
 		 * referenced via {@literal fieldReference}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Log}.
 		 */
 		public Log log(String fieldReference) {
 
@@ -236,7 +236,7 @@ public class ArithmeticOperators {
 		 * extracted by given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Log}.
 		 */
 		public Log log(AggregationExpression expression) {
 
@@ -249,7 +249,7 @@ public class ArithmeticOperators {
 		 * {@literal base}.
 		 *
 		 * @param base must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Log}.
 		 */
 		public Log log(Number base) {
 
@@ -264,7 +264,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that calculates the log base 10 for the associated number.
 		 *
-		 * @return
+		 * @return new instance of {@link Log10}.
 		 */
 		public Log10 log10() {
 			return usesFieldRef() ? Log10.log10ValueOf(fieldReference) : Log10.log10ValueOf(expression);
@@ -275,7 +275,7 @@ public class ArithmeticOperators {
 		 * remainder.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Mod}.
 		 */
 		public Mod mod(String fieldReference) {
 
@@ -288,7 +288,7 @@ public class ArithmeticOperators {
 		 * remainder.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Mod}.
 		 */
 		public Mod mod(AggregationExpression expression) {
 
@@ -301,7 +301,7 @@ public class ArithmeticOperators {
 		 * remainder.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Mod}.
 		 */
 		public Mod mod(Number value) {
 
@@ -317,7 +317,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that multiplies the associated number with another.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Multiply}.
 		 */
 		public Multiply multiplyBy(String fieldReference) {
 
@@ -329,7 +329,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that multiplies the associated number with another.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Multiply}.
 		 */
 		public Multiply multiplyBy(AggregationExpression expression) {
 
@@ -341,7 +341,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that multiplies the associated number with another.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Multiply}.
 		 */
 		public Multiply multiplyBy(Number value) {
 
@@ -357,7 +357,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that raises the associated number to the specified exponent.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Pow}.
 		 */
 		public Pow pow(String fieldReference) {
 
@@ -369,7 +369,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that raises the associated number to the specified exponent.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Pow}.
 		 */
 		public Pow pow(AggregationExpression expression) {
 
@@ -381,7 +381,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that raises the associated number to the specified exponent.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Pow}.
 		 */
 		public Pow pow(Number value) {
 
@@ -396,7 +396,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that calculates the square root of the associated number.
 		 *
-		 * @return
+		 * @return new instance of {@link Sqrt}.
 		 */
 		public Sqrt sqrt() {
 			return usesFieldRef() ? Sqrt.sqrtOf(fieldReference) : Sqrt.sqrtOf(expression);
@@ -406,7 +406,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that subtracts value of given from the associated number.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Subtract}.
 		 */
 		public Subtract subtract(String fieldReference) {
 
@@ -418,7 +418,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link AggregationExpression} that subtracts value of given from the associated number.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Subtract}.
 		 */
 		public Subtract subtract(AggregationExpression expression) {
 
@@ -429,8 +429,8 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that subtracts value from the associated number.
 		 *
-		 * @param value
-		 * @return
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Subtract}.
 		 */
 		public Subtract subtract(Number value) {
 
@@ -445,7 +445,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that truncates a number to its integer.
 		 *
-		 * @return
+		 * @return new instance of {@link Trunc}.
 		 */
 		public Trunc trunc() {
 			return usesFieldRef() ? Trunc.truncValueOf(fieldReference) : Trunc.truncValueOf(expression);
@@ -454,7 +454,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that calculates and returns the sum of numeric values.
 		 *
-		 * @return
+		 * @return new instance of {@link Sum}.
 		 */
 		public Sum sum() {
 			return usesFieldRef() ? AccumulatorOperators.Sum.sumOf(fieldReference)
@@ -464,7 +464,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the average value of the numeric values.
 		 *
-		 * @return
+		 * @return new instance of {@link Avg}.
 		 */
 		public Avg avg() {
 			return usesFieldRef() ? AccumulatorOperators.Avg.avgOf(fieldReference)
@@ -474,7 +474,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the maximum value.
 		 *
-		 * @return
+		 * @return new instance of {@link Max}.
 		 */
 		public Max max() {
 			return usesFieldRef() ? AccumulatorOperators.Max.maxOf(fieldReference)
@@ -484,7 +484,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the minimum value.
 		 *
-		 * @return
+		 * @return new instance of {@link Min}.
 		 */
 		public Min min() {
 			return usesFieldRef() ? AccumulatorOperators.Min.minOf(fieldReference)
@@ -494,7 +494,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that calculates the population standard deviation of the input values.
 		 *
-		 * @return
+		 * @return new instance of {@link StdDevPop}.
 		 */
 		public StdDevPop stdDevPop() {
 			return usesFieldRef() ? AccumulatorOperators.StdDevPop.stdDevPopOf(fieldReference)
@@ -504,7 +504,7 @@ public class ArithmeticOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that calculates the sample standard deviation of the input values.
 		 *
-		 * @return
+		 * @return new instance of {@link StdDevSamp}.
 		 */
 		public StdDevSamp stdDevSamp() {
 			return usesFieldRef() ? AccumulatorOperators.StdDevSamp.stdDevSampOf(fieldReference)
@@ -557,7 +557,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Abs}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Abs}.
 		 */
 		public static Abs absoluteValueOf(String fieldReference) {
 
@@ -569,7 +569,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Abs}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Abs}.
 		 */
 		public static Abs absoluteValueOf(AggregationExpression expression) {
 
@@ -581,7 +581,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Abs}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Abs}.
 		 */
 		public static Abs absoluteValueOf(Number value) {
 
@@ -610,7 +610,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Add}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Add}.
 		 */
 		public static Add valueOf(String fieldReference) {
 
@@ -622,7 +622,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Add}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Add}.
 		 */
 		public static Add valueOf(AggregationExpression expression) {
 
@@ -634,7 +634,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Add}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Add}.
 		 */
 		public static Add valueOf(Number value) {
 
@@ -642,18 +642,36 @@ public class ArithmeticOperators {
 			return new Add(Collections.singletonList(value));
 		}
 
+		/**
+		 * Add the value stored at the given field.
+		 * 
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Add}.
+		 */
 		public Add add(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Add(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Add the evaluation result of the given {@link AggregationExpression}.
+		 * 
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Add}.
+		 */
 		public Add add(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Add(append(expression));
 		}
 
+		/**
+		 * Add the given value.
+		 * 
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Add}.
+		 */
 		public Add add(Number value) {
 			return new Add(append(value));
 		}
@@ -679,7 +697,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Ceil}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ceil}.
 		 */
 		public static Ceil ceilValueOf(String fieldReference) {
 
@@ -691,7 +709,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Ceil}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ceil}.
 		 */
 		public static Ceil ceilValueOf(AggregationExpression expression) {
 
@@ -703,7 +721,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Ceil}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ceil}.
 		 */
 		public static Ceil ceilValueOf(Number value) {
 
@@ -732,7 +750,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Divide}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Divide}.
 		 */
 		public static Divide valueOf(String fieldReference) {
 
@@ -744,7 +762,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Divide}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Divide}.
 		 */
 		public static Divide valueOf(AggregationExpression expression) {
 
@@ -756,7 +774,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Divide}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Divide}.
 		 */
 		public static Divide valueOf(Number value) {
 
@@ -764,18 +782,36 @@ public class ArithmeticOperators {
 			return new Divide(Collections.singletonList(value));
 		}
 
+		/**
+		 * Divide by the value stored at the given field.
+		 * 
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Divide}.
+		 */
 		public Divide divideBy(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Divide(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Divide by the evaluation results of the given {@link AggregationExpression}.
+		 * 
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Divide}.
+		 */
 		public Divide divideBy(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Divide(append(expression));
 		}
 
+		/**
+		 * Divide by the given value.
+		 * 
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Divide}.
+		 */
 		public Divide divideBy(Number value) {
 			return new Divide(append(value));
 		}
@@ -801,7 +837,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Exp}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Exp}.
 		 */
 		public static Exp expValueOf(String fieldReference) {
 
@@ -813,7 +849,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Exp}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Exp}.
 		 */
 		public static Exp expValueOf(AggregationExpression expression) {
 
@@ -825,7 +861,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Exp}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Exp}.
 		 */
 		public static Exp expValueOf(Number value) {
 
@@ -854,7 +890,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Floor}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Floor}.
 		 */
 		public static Floor floorValueOf(String fieldReference) {
 
@@ -866,7 +902,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Floor}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Floor}.
 		 */
 		public static Floor floorValueOf(AggregationExpression expression) {
 
@@ -878,7 +914,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Floor}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Floor}.
 		 */
 		public static Floor floorValueOf(Number value) {
 
@@ -907,7 +943,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Ln}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ln}.
 		 */
 		public static Ln lnValueOf(String fieldReference) {
 
@@ -919,7 +955,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Ln}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ln}.
 		 */
 		public static Ln lnValueOf(AggregationExpression expression) {
 
@@ -931,7 +967,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Ln}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ln}.
 		 */
 		public static Ln lnValueOf(Number value) {
 
@@ -960,7 +996,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Min}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Log}.
 		 */
 		public static Log valueOf(String fieldReference) {
 
@@ -972,7 +1008,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Log}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Log}.
 		 */
 		public static Log valueOf(AggregationExpression expression) {
 
@@ -992,18 +1028,36 @@ public class ArithmeticOperators {
 			return new Log(Collections.singletonList(value));
 		}
 
+		/**
+		 * Use the value stored at the given field as log base.
+		 * 
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Log}.
+		 */
 		public Log log(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Log(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Use the evaluated value of the given {@link AggregationExpression} as log base.
+		 * 
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Log}.
+		 */
 		public Log log(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Log(append(expression));
 		}
 
+		/**
+		 * Use the given value as log base.
+		 * 
+		 * @param base must not be {@literal null}.
+		 * @return new instance of {@link Log}.
+		 */
 		public Log log(Number base) {
 			return new Log(append(base));
 		}
@@ -1029,7 +1083,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Log10}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Log10}.
 		 */
 		public static Log10 log10ValueOf(String fieldReference) {
 
@@ -1041,7 +1095,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Log10}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Log10}.
 		 */
 		public static Log10 log10ValueOf(AggregationExpression expression) {
 
@@ -1053,7 +1107,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Log10}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Log10}.
 		 */
 		public static Log10 log10ValueOf(Number value) {
 
@@ -1082,7 +1136,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Mod}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Mod}.
 		 */
 		public static Mod valueOf(String fieldReference) {
 
@@ -1094,7 +1148,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Mod}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Mod}.
 		 */
 		public static Mod valueOf(AggregationExpression expression) {
 
@@ -1106,7 +1160,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Mod}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Mod}.
 		 */
 		public static Mod valueOf(Number value) {
 
@@ -1114,18 +1168,36 @@ public class ArithmeticOperators {
 			return new Mod(Collections.singletonList(value));
 		}
 
+		/**
+		 * Use the value stored at the given field as mod base.
+		 * 
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Mod}.
+		 */
 		public Mod mod(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Mod(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Use evaluated value of the given {@link AggregationExpression} as mod base.
+		 * 
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Mod}.
+		 */
 		public Mod mod(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Mod(append(expression));
 		}
 
+		/**
+		 * Use the given value as mod base.
+		 * 
+		 * @param base must not be {@literal null}.
+		 * @return new instance of {@link Mod}.
+		 */
 		public Mod mod(Number base) {
 			return new Mod(append(base));
 		}
@@ -1151,7 +1223,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Multiply}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Multiply}.
 		 */
 		public static Multiply valueOf(String fieldReference) {
 
@@ -1163,7 +1235,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Multiply}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Multiply}.
 		 */
 		public static Multiply valueOf(AggregationExpression expression) {
 
@@ -1175,7 +1247,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Multiply}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Multiply}.
 		 */
 		public static Multiply valueOf(Number value) {
 
@@ -1183,18 +1255,36 @@ public class ArithmeticOperators {
 			return new Multiply(Collections.singletonList(value));
 		}
 
+		/**
+		 * Multiply by the value stored at the given field.
+		 * 
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Multiply}.
+		 */
 		public Multiply multiplyBy(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Multiply(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Multiply by the evaluated value of the given {@link AggregationExpression}.
+		 * 
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Multiply}.
+		 */
 		public Multiply multiplyBy(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Multiply(append(expression));
 		}
 
+		/**
+		 * Multiply by the given value.
+		 * 
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Multiply}.
+		 */
 		public Multiply multiplyBy(Number value) {
 			return new Multiply(append(value));
 		}
@@ -1252,18 +1342,36 @@ public class ArithmeticOperators {
 			return new Pow(Collections.singletonList(value));
 		}
 
+		/**
+		 * Pow by the value stored at the given field.
+		 * 
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Pow}.
+		 */
 		public Pow pow(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Pow(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Pow by the evaluated value of the given {@link AggregationExpression}.
+		 * 
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Pow}.
+		 */
 		public Pow pow(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Pow(append(expression));
 		}
 
+		/**
+		 * Pow by the given value.
+		 * 
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Pow}.
+		 */
 		public Pow pow(Number value) {
 			return new Pow(append(value));
 		}
@@ -1289,7 +1397,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Sqrt}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sqrt}.
 		 */
 		public static Sqrt sqrtOf(String fieldReference) {
 
@@ -1301,7 +1409,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Sqrt}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sqrt}.
 		 */
 		public static Sqrt sqrtOf(AggregationExpression expression) {
 
@@ -1313,7 +1421,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Sqrt}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Sqrt}.
 		 */
 		public static Sqrt sqrtOf(Number value) {
 
@@ -1342,7 +1450,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Subtract}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Subtract}.
 		 */
 		public static Subtract valueOf(String fieldReference) {
 
@@ -1354,7 +1462,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Subtract}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Subtract}.
 		 */
 		public static Subtract valueOf(AggregationExpression expression) {
 
@@ -1366,7 +1474,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Subtract}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Subtract}.
 		 */
 		public static Subtract valueOf(Number value) {
 
@@ -1374,18 +1482,36 @@ public class ArithmeticOperators {
 			return new Subtract(Collections.singletonList(value));
 		}
 
+		/**
+		 * Subtract the value stored at the given field.
+		 * 
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Pow}.
+		 */
 		public Subtract subtract(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Subtract(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Subtract the evaluated value of the given {@link AggregationExpression}.
+		 * 
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Pow}.
+		 */
 		public Subtract subtract(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Subtract(append(expression));
 		}
 
+		/**
+		 * Subtract the given value.
+		 * 
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Pow}.
+		 */
 		public Subtract subtract(Number value) {
 			return new Subtract(append(value));
 		}
@@ -1411,7 +1537,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Trunc}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Trunc}.
 		 */
 		public static Trunc truncValueOf(String fieldReference) {
 
@@ -1423,7 +1549,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Trunc}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Trunc}.
 		 */
 		public static Trunc truncValueOf(AggregationExpression expression) {
 
@@ -1435,7 +1561,7 @@ public class ArithmeticOperators {
 		 * Creates new {@link Trunc}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Trunc}.
 		 */
 		public static Trunc truncValueOf(Number value) {
 
@@ -1502,7 +1628,7 @@ public class ArithmeticOperators {
 		/**
 		 * The place to round to. Can be between -20 and 100, exclusive.
 		 *
-		 * @param place
+		 * @param place value between -20 and 100, exclusive.
 		 * @return new instance of {@link Round}.
 		 */
 		public Round place(int place) {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ArrayOperators.java

@@ -43,7 +43,7 @@ public class ArrayOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ArrayOperatorFactory}.
 	 */
 	public static ArrayOperatorFactory arrayOf(String fieldReference) {
 		return new ArrayOperatorFactory(fieldReference);
@@ -53,7 +53,7 @@ public class ArrayOperators {
 	 * Take the array referenced resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ArrayOperatorFactory}.
 	 */
 	public static ArrayOperatorFactory arrayOf(AggregationExpression expression) {
 		return new ArrayOperatorFactory(expression);
@@ -123,8 +123,8 @@ public class ArrayOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated array and returns the element at the
 		 * specified array {@literal position}.
 		 *
-		 * @param position
-		 * @return
+		 * @param position the element index.
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public ArrayElemAt elementAt(int position) {
 			return createArrayElemAt().elementAt(position);
@@ -135,7 +135,7 @@ public class ArrayOperators {
 		 * resulting form the given {@literal expression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public ArrayElemAt elementAt(AggregationExpression expression) {
 
@@ -148,7 +148,7 @@ public class ArrayOperators {
 		 * defined by the referenced {@literal field}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public ArrayElemAt elementAt(String fieldReference) {
 
@@ -170,7 +170,7 @@ public class ArrayOperators {
 		 * {@literal arrayFieldReference} to it.
 		 *
 		 * @param arrayFieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ConcatArrays}.
 		 */
 		public ConcatArrays concat(String arrayFieldReference) {
 
@@ -183,7 +183,7 @@ public class ArrayOperators {
 		 * the given {@literal expression} to it.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ConcatArrays}.
 		 */
 		public ConcatArrays concat(AggregationExpression expression) {
 
@@ -204,7 +204,7 @@ public class ArrayOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated array and selects a subset of the array to
 		 * return based on the specified condition.
 		 *
-		 * @return
+		 * @return new instance of {@link AsBuilder} to create a {@link Filter}.
 		 */
 		public AsBuilder filter() {
 
@@ -219,7 +219,7 @@ public class ArrayOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated array and an check if its an array.
 		 *
-		 * @return
+		 * @return new instance of {@link IsArray}.
 		 */
 		public IsArray isArray() {
 
@@ -231,7 +231,7 @@ public class ArrayOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated array and retrieves its length.
 		 *
-		 * @return
+		 * @return new instance of {@link Size}.
 		 */
 		public Size length() {
 
@@ -245,7 +245,7 @@ public class ArrayOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated array and selects a subset from it.
 		 *
-		 * @return
+		 * @return new instance of {@link Slice}.
 		 */
 		public Slice slice() {
 
@@ -261,7 +261,7 @@ public class ArrayOperators {
 		 * value and returns the array index (zero-based) of the first occurrence.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfArray}.
 		 */
 		public IndexOfArray indexOf(Object value) {
 
@@ -276,7 +276,7 @@ public class ArrayOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns an array with the elements in reverse order.
 		 *
-		 * @return
+		 * @return new instance of {@link ReverseArray}.
 		 */
 		public ReverseArray reverse() {
 
@@ -293,7 +293,7 @@ public class ArrayOperators {
 		 * an array and combines them into a single value.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReduceInitialValueBuilder} to create {@link Reduce}.
 		 */
 		public ArrayOperatorFactory.ReduceInitialValueBuilder reduce(AggregationExpression expression) {
 
@@ -305,8 +305,8 @@ public class ArrayOperators {
 		 * Start creating new {@link AggregationExpression} that applies an {@link AggregationExpression} to each element in
 		 * an array and combines them into a single value.
 		 *
-		 * @param expressions
-		 * @return
+		 * @param expressions must not be {@literal null}.
+		 * @return new instance of {@link ReduceInitialValueBuilder} to create {@link Reduce}.
 		 */
 		public ArrayOperatorFactory.ReduceInitialValueBuilder reduce(PropertyExpression... expressions) {
 
@@ -320,7 +320,7 @@ public class ArrayOperators {
 		 * the second input array, etc.
 		 *
 		 * @param arrays must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip zipWith(Object... arrays) {
 
@@ -336,7 +336,7 @@ public class ArrayOperators {
 		 * associated array.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link In}.
 		 */
 		public In containsValue(Object value) {
 
@@ -413,7 +413,7 @@ public class ArrayOperators {
 		 * Creates new {@link ArrayElemAt}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public static ArrayElemAt arrayOf(String fieldReference) {
 
@@ -425,7 +425,7 @@ public class ArrayOperators {
 		 * Creates new {@link ArrayElemAt}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ArrayElemAt}.
 		 */
 		public static ArrayElemAt arrayOf(AggregationExpression expression) {
 
@@ -446,16 +446,34 @@ public class ArrayOperators {
 			return new ArrayElemAt(Collections.singletonList(values));
 		}
 
+		/**
+		 * Use the element with given index number.
+		 *
+		 * @param index the index number
+		 * @return new instance of {@link ArrayElemAt}.
+		 */
 		public ArrayElemAt elementAt(int index) {
 			return new ArrayElemAt(append(index));
 		}
 
+		/**
+		 * Use the element at the index number evaluated from the given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link ArrayElemAt}.
+		 */
 		public ArrayElemAt elementAt(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new ArrayElemAt(append(expression));
 		}
 
+		/**
+		 * Use the element at the index number traken from the given field.
+		 *
+		 * @param arrayFieldReference the field name.
+		 * @return new instance of {@link ArrayElemAt}.
+		 */
 		public ArrayElemAt elementAt(String arrayFieldReference) {
 
 			Assert.notNull(arrayFieldReference, "ArrayReference must not be null!");
@@ -483,7 +501,7 @@ public class ArrayOperators {
 		 * Creates new {@link ConcatArrays}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ConcatArrays}.
 		 */
 		public static ConcatArrays arrayOf(String fieldReference) {
 
@@ -495,7 +513,7 @@ public class ArrayOperators {
 		 * Creates new {@link ConcatArrays}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ConcatArrays}.
 		 */
 		public static ConcatArrays arrayOf(AggregationExpression expression) {
 
@@ -516,12 +534,24 @@ public class ArrayOperators {
 			return new ConcatArrays(Collections.singletonList(values));
 		}
 
+		/**
+		 * Concat with the array stored at the given field.
+		 *
+		 * @param arrayFieldReference must not be {@literal null}.
+		 * @return new instance of {@link ConcatArrays}.
+		 */
 		public ConcatArrays concat(String arrayFieldReference) {
 
 			Assert.notNull(arrayFieldReference, "ArrayFieldReference must not be null!");
 			return new ConcatArrays(append(Fields.field(arrayFieldReference)));
 		}
 
+		/**
+		 * Concat with the array resulting from the given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link ConcatArrays}.
+		 */
 		public ConcatArrays concat(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
@@ -574,7 +604,7 @@ public class ArrayOperators {
 		 * Set the {@literal values} to apply the {@code $filter} to.
 		 *
 		 * @param values must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AsBuilder} to create the {@link Filter}.
 		 */
 		public static AsBuilder filter(List<?> values) {
 
@@ -651,7 +681,7 @@ public class ArrayOperators {
 			 * Set the {@literal variableName} for the elements in the input array.
 			 *
 			 * @param variableName must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			ConditionBuilder as(String variableName);
 		}
@@ -665,7 +695,7 @@ public class ArrayOperators {
 			 * Set the {@link AggregationExpression} that determines whether to include the element in the resulting array.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			Filter by(AggregationExpression expression);
 
@@ -673,7 +703,7 @@ public class ArrayOperators {
 			 * Set the {@literal expression} that determines whether to include the element in the resulting array.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			Filter by(String expression);
 
@@ -681,7 +711,7 @@ public class ArrayOperators {
 			 * Set the {@literal expression} that determines whether to include the element in the resulting array.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			Filter by(Document expression);
 		}
@@ -700,7 +730,7 @@ public class ArrayOperators {
 			/**
 			 * Creates new {@link InputBuilder}.
 			 *
-			 * @return
+			 * @return new instance of {@link FilterExpressionBuilder}.
 			 */
 			public static InputBuilder newBuilder() {
 				return new FilterExpressionBuilder();
@@ -800,7 +830,7 @@ public class ArrayOperators {
 		 * Creates new {@link IsArray}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsArray}.
 		 */
 		public static IsArray isArray(String fieldReference) {
 
@@ -812,7 +842,7 @@ public class ArrayOperators {
 		 * Creates new {@link IsArray}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsArray}.
 		 */
 		public static IsArray isArray(AggregationExpression expression) {
 
@@ -841,7 +871,7 @@ public class ArrayOperators {
 		 * Creates new {@link Size}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Size}.
 		 */
 		public static Size lengthOfArray(String fieldReference) {
 
@@ -853,7 +883,7 @@ public class ArrayOperators {
 		 * Creates new {@link Size}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Size}.
 		 */
 		public static Size lengthOfArray(AggregationExpression expression) {
 
@@ -895,7 +925,7 @@ public class ArrayOperators {
 		 * Creates new {@link Slice}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Slice}.
 		 */
 		public static Slice sliceArrayOf(String fieldReference) {
 
@@ -907,7 +937,7 @@ public class ArrayOperators {
 		 * Creates new {@link Slice}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Slice}.
 		 */
 		public static Slice sliceArrayOf(AggregationExpression expression) {
 
@@ -928,10 +958,22 @@ public class ArrayOperators {
 			return new Slice(Collections.singletonList(values));
 		}
 
+		/**
+		 * Slice the number of elements.
+		 *
+		 * @param nrElements elements to slice.
+		 * @return new instance of {@link Slice}.
+		 */
 		public Slice itemCount(int nrElements) {
 			return new Slice(append(nrElements));
 		}
 
+		/**
+		 * Slice using offset and count.
+		 *
+		 * @param position the start position
+		 * @return new instance of {@link SliceElementsBuilder} to create {@link Slice}.
+		 */
 		public SliceElementsBuilder offset(final int position) {
 
 			return new SliceElementsBuilder() {
@@ -978,7 +1020,7 @@ public class ArrayOperators {
 		 * Start creating new {@link IndexOfArray}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfArray}.
 		 */
 		public static IndexOfArrayBuilder arrayOf(String fieldReference) {
 
@@ -990,7 +1032,7 @@ public class ArrayOperators {
 		 * Start creating new {@link IndexOfArray}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfArray}.
 		 */
 		public static IndexOfArrayBuilder arrayOf(AggregationExpression expression) {
 
@@ -1002,7 +1044,7 @@ public class ArrayOperators {
 		 * Start creating new {@link IndexOfArray}.
 		 *
 		 * @param values must not be {@literal null}.
-		 * @return new instance of {@link IndexOfArray}.
+		 * @return new instance of {@link IndexOfArrayBuilder} to create {@link IndexOfArray}.
 		 * @since 2.2
 		 */
 		public static IndexOfArrayBuilder arrayOf(Collection<?> values) {
@@ -1011,6 +1053,12 @@ public class ArrayOperators {
 			return new IndexOfArrayBuilder(values);
 		}
 
+		/**
+		 * Lookup within a given range.
+		 *
+		 * @param range the lookup range.
+		 * @return new instance of {@link IndexOfArray}.
+		 */
 		public IndexOfArray within(Range<Long> range) {
 			return new IndexOfArray(append(AggregationUtils.toRangeValues(range)));
 		}
@@ -1030,7 +1078,7 @@ public class ArrayOperators {
 			 * Set the {@literal value} to check for its index in the array.
 			 *
 			 * @param value must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfArray}.
 			 */
 			public IndexOfArray indexOf(Object value) {
 
@@ -1060,7 +1108,7 @@ public class ArrayOperators {
 		 * Start creating new {@link RangeOperator}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link RangeOperatorBuilder} to create {@link RangeOperator}.
 		 */
 		public static RangeOperatorBuilder rangeStartingAt(String fieldReference) {
 			return new RangeOperatorBuilder(Fields.field(fieldReference));
@@ -1070,7 +1118,7 @@ public class ArrayOperators {
 		 * Start creating new {@link RangeOperator}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link RangeOperatorBuilder} to create {@link RangeOperator}.
 		 */
 		public static RangeOperatorBuilder rangeStartingAt(AggregationExpression expression) {
 			return new RangeOperatorBuilder(expression);
@@ -1080,7 +1128,7 @@ public class ArrayOperators {
 		 * Start creating new {@link RangeOperator}.
 		 *
 		 * @param value
-		 * @return
+		 * @return new instance of {@link RangeOperator}.
 		 */
 		public static RangeOperatorBuilder rangeStartingAt(long value) {
 			return new RangeOperatorBuilder(value);
@@ -1102,7 +1150,7 @@ public class ArrayOperators {
 			 * Creates new {@link RangeOperator}.
 			 *
 			 * @param index
-			 * @return
+			 * @return new instance of {@link RangeOperator}.
 			 */
 			public RangeOperator to(long index) {
 				return new RangeOperator(Arrays.asList(startPoint, index));
@@ -1112,7 +1160,7 @@ public class ArrayOperators {
 			 * Creates new {@link RangeOperator}.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link RangeOperator}.
 			 */
 			public RangeOperator to(AggregationExpression expression) {
 				return new RangeOperator(Arrays.asList(startPoint, expression));
@@ -1122,7 +1170,7 @@ public class ArrayOperators {
 			 * Creates new {@link RangeOperator}.
 			 *
 			 * @param fieldReference must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link RangeOperator}.
 			 */
 			public RangeOperator to(String fieldReference) {
 				return new RangeOperator(Arrays.asList(startPoint, Fields.field(fieldReference)));
@@ -1150,7 +1198,7 @@ public class ArrayOperators {
 		 * Creates new {@link ReverseArray} given {@literal fieldReference}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReverseArray}.
 		 */
 		public static ReverseArray reverseArrayOf(String fieldReference) {
 			return new ReverseArray(Fields.field(fieldReference));
@@ -1160,7 +1208,7 @@ public class ArrayOperators {
 		 * Creates new {@link ReverseArray} given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReverseArray}.
 		 */
 		public static ReverseArray reverseArrayOf(AggregationExpression expression) {
 			return new ReverseArray(expression);
@@ -1239,7 +1287,7 @@ public class ArrayOperators {
 		 * Start creating new {@link Reduce}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link InitialValueBuilder} to create {@link Reduce}.
 		 */
 		public static InitialValueBuilder arrayOf(final String fieldReference) {
 
@@ -1267,7 +1315,7 @@ public class ArrayOperators {
 							Assert.notNull(expressions, "PropertyExpressions must not be null");
 
 							return new Reduce(Fields.field(fieldReference), initialValue,
-									Arrays.<AggregationExpression> asList(expressions));
+									Arrays.<AggregationExpression>asList(expressions));
 						}
 					};
 				}
@@ -1278,7 +1326,7 @@ public class ArrayOperators {
 		 * Start creating new {@link Reduce}.
 		 *
 		 * @param arrayValueExpression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link InitialValueBuilder} to create {@link Reduce}.
 		 */
 		public static InitialValueBuilder arrayOf(final AggregationExpression arrayValueExpression) {
 
@@ -1318,7 +1366,7 @@ public class ArrayOperators {
 			 * Define the initial cumulative value set before in is applied to the first element of the input array.
 			 *
 			 * @param initialValue must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			ReduceBuilder withInitialValue(Object initialValue);
 		}
@@ -1335,7 +1383,7 @@ public class ArrayOperators {
 			 * {@link Variable#VALUE} are available.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link Reduce}.
 			 */
 			Reduce reduce(AggregationExpression expression);
 
@@ -1346,7 +1394,7 @@ public class ArrayOperators {
 			 * {@link Variable#VALUE} are available.
 			 *
 			 * @param expressions must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link Reduce}.
 			 */
 			Reduce reduce(PropertyExpression... expressions);
 		}
@@ -1372,7 +1420,7 @@ public class ArrayOperators {
 			 * Define a result property for an {@link AggregationExpression} used in {@link Reduce}.
 			 *
 			 * @param name must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link AsBuilder} to create {@link Reduce}.
 			 */
 			public static AsBuilder property(final String name) {
 
@@ -1402,7 +1450,7 @@ public class ArrayOperators {
 				 * Set the {@link AggregationExpression} resulting in the properties value.
 				 *
 				 * @param expression must not be {@literal null}.
-				 * @return
+				 * @return never {@literal null}.
 				 */
 				PropertyExpression definedAs(AggregationExpression expression);
 			}
@@ -1459,7 +1507,7 @@ public class ArrayOperators {
 			 * eg. {@code $$value.product}
 			 *
 			 * @param property must not be {@literal null}.
-			 * @return
+			 * @return never {@literal null}.
 			 */
 			public Field referringTo(final String property) {
 
@@ -1508,7 +1556,7 @@ public class ArrayOperators {
 		 * Start creating new {@link Zip}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ZipBuilder} to create {@link Zip}.
 		 */
 		public static ZipBuilder arrayOf(String fieldReference) {
 
@@ -1520,7 +1568,7 @@ public class ArrayOperators {
 		 * Start creating new {@link Zip}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ZipBuilder} to create {@link Zip}.
 		 */
 		public static ZipBuilder arrayOf(AggregationExpression expression) {
 
@@ -1544,7 +1592,7 @@ public class ArrayOperators {
 		/**
 		 * Create new {@link Zip} and set the {@code useLongestLength} property to {@literal true}.
 		 *
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip useLongestLength() {
 			return new Zip(append("useLongestLength", true));
@@ -1554,7 +1602,7 @@ public class ArrayOperators {
 		 * Optionally provide a default value.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip defaultTo(String fieldReference) {
 
@@ -1566,7 +1614,7 @@ public class ArrayOperators {
 		 * Optionally provide a default value.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip defaultTo(AggregationExpression expression) {
 
@@ -1578,7 +1626,7 @@ public class ArrayOperators {
 		 * Optionally provide a default value.
 		 *
 		 * @param array must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Zip}.
 		 */
 		public Zip defaultTo(Object[] array) {
 
@@ -1602,7 +1650,7 @@ public class ArrayOperators {
 			 * array, etc.
 			 *
 			 * @param arrays arrays to zip the referenced one with. must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link Zip}.
 			 */
 			public Zip zip(Object... arrays) {
 
@@ -1616,7 +1664,7 @@ public class ArrayOperators {
 					}
 				}
 
-				return new Zip(Collections.<String, Object> singletonMap("inputs", sourceArrays));
+				return new Zip(Collections.<String, Object>singletonMap("inputs", sourceArrays));
 			}
 		}
 	}
@@ -1627,7 +1675,7 @@ public class ArrayOperators {
 	 * @author Christoph Strobl
 	 * @author Shashank Sharma
 	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/in/">https://docs.mongodb.com/manual/reference/operator/aggregation/in/</a>
+	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/in/">https://docs.mongodb.com/manual/reference/operator/aggregation/in/</a>
 	 * @since 2.2
 	 */
 	public static class In extends AbstractAggregationExpression {
@@ -1645,7 +1693,7 @@ public class ArrayOperators {
 		 * Start creating {@link In}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link InBuilder} to create {@link In}.
 		 */
 		public static InBuilder arrayOf(String fieldReference) {
 
@@ -1662,7 +1710,7 @@ public class ArrayOperators {
 		 * Start creating {@link In}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link InBuilder} to create {@link In}.
 		 */
 		public static InBuilder arrayOf(AggregationExpression expression) {
 
@@ -1704,7 +1752,7 @@ public class ArrayOperators {
 			 * Set the {@literal value} to check for existence in the array.
 			 *
 			 * @param value must not be {@literal value}.
-			 * @return
+			 * @return new instance of {@link In}.
 			 */
 			In containsValue(Object value);
 		}
@@ -1716,7 +1764,7 @@ public class ArrayOperators {
 	 *
 	 * @author Christoph Strobl
 	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/">https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/</a>
+	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/">https://docs.mongodb.com/manual/reference/operator/aggregation/arrayToObject/</a>
 	 * @since 2.1
 	 */
 	public static class ArrayToObject extends AbstractAggregationExpression {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/BooleanOperators.java

@@ -34,7 +34,7 @@ public class BooleanOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BooleanOperatorFactory}.
 	 */
 	public static BooleanOperatorFactory valueOf(String fieldReference) {
 		return new BooleanOperatorFactory(fieldReference);
@@ -44,7 +44,7 @@ public class BooleanOperators {
 	 * Take the value resulting of the given {@link AggregationExpression}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BooleanOperatorFactory}.
 	 */
 	public static BooleanOperatorFactory valueOf(AggregationExpression fieldReference) {
 		return new BooleanOperatorFactory(fieldReference);
@@ -55,7 +55,7 @@ public class BooleanOperators {
 	 * opposite boolean value.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Not}.
 	 */
 	public static Not not(String fieldReference) {
 		return Not.not(fieldReference);
@@ -66,7 +66,7 @@ public class BooleanOperators {
 	 * and returns the opposite boolean value.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Not}.
 	 */
 	public static Not not(AggregationExpression expression) {
 		return Not.not(expression);
@@ -109,7 +109,7 @@ public class BooleanOperators {
 		 * all of the expressions are {@literal true}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And and(AggregationExpression expression) {
 
@@ -122,7 +122,7 @@ public class BooleanOperators {
 		 * all of the expressions are {@literal true}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And and(String fieldReference) {
 
@@ -139,7 +139,7 @@ public class BooleanOperators {
 		 * any of the expressions are {@literal true}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or or(AggregationExpression expression) {
 
@@ -152,7 +152,7 @@ public class BooleanOperators {
 		 * any of the expressions are {@literal true}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or or(String fieldReference) {
 
@@ -167,7 +167,7 @@ public class BooleanOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that evaluates a boolean and returns the opposite boolean value.
 		 *
-		 * @return
+		 * @return new instance of {@link Not}.
 		 */
 		public Not not() {
 			return usesFieldRef() ? Not.not(fieldReference) : Not.not(expression);
@@ -198,8 +198,8 @@ public class BooleanOperators {
 		 * Creates new {@link And} that evaluates one or more expressions and returns {@literal true} if all of the
 		 * expressions are {@literal true}.
 		 *
-		 * @param expressions
-		 * @return
+		 * @param expressions must not be {@literal null}.
+		 * @return new instance of {@link And}.
 		 */
 		public static And and(Object... expressions) {
 			return new And(Arrays.asList(expressions));
@@ -209,7 +209,7 @@ public class BooleanOperators {
 		 * Creates new {@link And} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And andExpression(AggregationExpression expression) {
 
@@ -221,7 +221,7 @@ public class BooleanOperators {
 		 * Creates new {@link And} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And andField(String fieldReference) {
 
@@ -233,7 +233,7 @@ public class BooleanOperators {
 		 * Creates new {@link And} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link And}.
 		 */
 		public And andValue(Object value) {
 
@@ -263,7 +263,7 @@ public class BooleanOperators {
 		 * expressions are {@literal true}.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public static Or or(Object... expressions) {
 
@@ -275,7 +275,7 @@ public class BooleanOperators {
 		 * Creates new {@link Or} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or orExpression(AggregationExpression expression) {
 
@@ -287,7 +287,7 @@ public class BooleanOperators {
 		 * Creates new {@link Or} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or orField(String fieldReference) {
 
@@ -299,7 +299,7 @@ public class BooleanOperators {
 		 * Creates new {@link Or} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Or}.
 		 */
 		public Or orValue(Object value) {
 
@@ -329,7 +329,7 @@ public class BooleanOperators {
 		 * value.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Not}.
 		 */
 		public static Not not(String fieldReference) {
 
@@ -342,7 +342,7 @@ public class BooleanOperators {
 		 * returns the opposite boolean value.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Not}.
 		 */
 		public static Not not(AggregationExpression expression) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/BucketAutoOperation.java

@@ -15,12 +15,11 @@
  */
 package org.springframework.data.mongodb.core.aggregation;
 
+import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.BucketAutoOperation.BucketAutoOperationOutputBuilder;
 import org.springframework.data.mongodb.core.aggregation.BucketOperationSupport.OutputBuilder;
 import org.springframework.util.Assert;
 
-import org.bson.Document;
-
 /**
  * Encapsulates the aggregation framework {@code $bucketAuto}-operation. <br />
  * Bucket stage is typically used with {@link Aggregation} and {@code $facet}. Categorizes incoming documents into a
@@ -106,14 +105,23 @@ public class BucketAutoOperation extends BucketOperationSupport<BucketAutoOperat
 
 		options.putAll(super.toDocument(context));
 
-		return new Document("$bucketAuto", options);
+		return new Document(getOperator(), options);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$bucketAuto";
 	}
 
 	/**
 	 * Configures a number of bucket {@literal buckets} and return a new {@link BucketAutoOperation}.
 	 *
 	 * @param buckets must be a positive number.
-	 * @return
+	 * @return new instance of {@link BucketAutoOperation}.
 	 */
 	public BucketAutoOperation withBuckets(int buckets) {
 
@@ -128,7 +136,7 @@ public class BucketAutoOperation extends BucketOperationSupport<BucketAutoOperat
 	 * Use either predefined {@link Granularities} or provide a own one.
 	 *
 	 * @param granularity must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BucketAutoOperation}.
 	 */
 	public BucketAutoOperation withGranularity(Granularity granularity) {
 
@@ -209,7 +217,7 @@ public class BucketAutoOperation extends BucketOperationSupport<BucketAutoOperat
 		 *
 		 * @param expression must not be {@literal null}.
 		 * @param operation must not be {@literal null}.
-		 * @param parameters
+		 * @param parameters must not be {@literal null}.
 		 */
 		protected ExpressionBucketAutoOperationBuilder(String expression, BucketAutoOperation operation,
 				Object[] parameters) {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/BucketOperation.java

@@ -20,21 +20,19 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
+import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.BucketOperation.BucketOperationOutputBuilder;
 import org.springframework.util.Assert;
 
-import org.bson.Document;
-
 /**
  * Encapsulates the aggregation framework {@code $bucket}-operation. <br />
- *
  * Bucket stage is typically used with {@link Aggregation} and {@code $facet}. Categorizes incoming documents into
  * groups, called buckets, based on a specified expression and bucket boundaries. <br />
- *
  * We recommend to use the static factory method {@link Aggregation#bucket(String)} instead of creating instances of
  * this class directly.
  *
- * @see <a href="https://docs.mongodb.org/manual/reference/aggregation/bucket/">https://docs.mongodb.org/manual/reference/aggregation/bucket/</a>
+ * @see <a href=
+ *      "https://docs.mongodb.org/manual/reference/aggregation/bucket/">https://docs.mongodb.org/manual/reference/aggregation/bucket/</a>
  * @see BucketOperationSupport
  * @author Mark Paluch
  * @since 1.10
@@ -103,14 +101,23 @@ public class BucketOperation extends BucketOperationSupport<BucketOperation, Buc
 
 		options.putAll(super.toDocument(context));
 
-		return new Document("$bucket", options);
+		return new Document(getOperator(), options);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$bucket";
 	}
 
 	/**
 	 * Configures a default bucket {@literal literal} and return a new {@link BucketOperation}.
 	 *
 	 * @param literal must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BucketOperation}.
 	 */
 	public BucketOperation withDefaultBucket(Object literal) {
 
@@ -123,7 +130,7 @@ public class BucketOperation extends BucketOperationSupport<BucketOperation, Buc
 	 * preserved and the new {@literal boundaries} are appended.
 	 *
 	 * @param boundaries must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link BucketOperation}.
 	 */
 	public BucketOperation withBoundaries(Object... boundaries) {
 
@@ -204,12 +211,12 @@ public class BucketOperation extends BucketOperationSupport<BucketOperation, Buc
 			extends ExpressionBucketOperationBuilderSupport<BucketOperationOutputBuilder, BucketOperation> {
 
 		/**
-		 * Creates a new {@link ExpressionBucketOperationBuilderSupport} for the given value, {@link BucketOperation}
-		 * and parameters.
+		 * Creates a new {@link ExpressionBucketOperationBuilderSupport} for the given value, {@link BucketOperation} and
+		 * parameters.
 		 *
 		 * @param expression must not be {@literal null}.
 		 * @param operation must not be {@literal null}.
-		 * @param parameters
+		 * @param parameters must not be {@literal null}.
 		 */
 		protected ExpressionBucketOperationBuilder(String expression, BucketOperation operation, Object[] parameters) {
 			super(expression, operation, parameters);

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/BucketOperationSupport.java

@@ -21,14 +21,13 @@ import java.util.Collection;
 import java.util.Collections;
 import java.util.List;
 
+import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.BucketOperationSupport.OutputBuilder;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder;
 import org.springframework.expression.spel.ast.Projection;
 import org.springframework.util.Assert;
 
-import org.bson.Document;
-
 /**
  * Base class for bucket operations that support output expressions the aggregation framework. <br />
  * Bucket stages collect documents into buckets and can contribute output fields. <br />
@@ -104,7 +103,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 	 *
 	 * @param expression the SpEL expression, must not be {@literal null} or empty.
 	 * @param params must not be {@literal null}
-	 * @return
+	 * @return new instance of {@link ExpressionBucketOperationBuilderSupport} to create {@link BucketOperation}.
 	 */
 	public abstract ExpressionBucketOperationBuilderSupport<B, T> andOutputExpression(String expression,
 			Object... params);
@@ -114,7 +113,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 	 * resulting bucket documents.
 	 *
 	 * @param expression the SpEL expression, must not be {@literal null} or empty.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public abstract B andOutput(AggregationExpression expression);
 
@@ -124,14 +123,14 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 	 * {@literal fieldName}.
 	 *
 	 * @param fieldName must not be {@literal null} or empty.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public abstract B andOutput(String fieldName);
 
 	/**
 	 * Creates a new {@link BucketOperationSupport} given to add a count field to the resulting bucket documents.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public B andOutputCount() {
 		return andOutput(new AggregationExpression() {
@@ -231,7 +230,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 * Generates a builder for a {@code $sum}-expression. <br />
 		 * Count expressions are emulated via {@code $sum: 1}.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B count() {
 			return sum(1);
@@ -240,7 +239,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for a {@code $sum}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B sum() {
 			return apply(Accumulators.SUM);
@@ -249,8 +248,8 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for a {@code $sum}-expression for the given {@literal value}.
 		 *
-		 * @param value
-		 * @return
+		 * @param value must not be {@literal null}.
+		 * @return never {@literal null}.
 		 */
 		public B sum(Number value) {
 			return apply(new OperationOutput(Accumulators.SUM.getMongoOperator(), Collections.singleton(value)));
@@ -259,7 +258,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $last}-expression for the current value..
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B last() {
 			return apply(Accumulators.LAST);
@@ -268,7 +267,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for a {@code $first}-expression the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B first() {
 			return apply(Accumulators.FIRST);
@@ -277,8 +276,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $avg}-expression for the current value.
 		 *
-		 * @param reference
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B avg() {
 			return apply(Accumulators.AVG);
@@ -287,7 +285,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $min}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B min() {
 			return apply(Accumulators.MIN);
@@ -296,7 +294,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $max}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B max() {
 			return apply(Accumulators.MAX);
@@ -305,7 +303,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $push}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B push() {
 			return apply(Accumulators.PUSH);
@@ -314,7 +312,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Generates a builder for an {@code $addToSet}-expression for the current value.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B addToSet() {
 			return apply(Accumulators.ADDTOSET);
@@ -325,7 +323,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 *
 		 * @param operation the operation name, must not be {@literal null} or empty.
 		 * @param values must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public B apply(String operation, Object... values) {
 
@@ -342,7 +340,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 * Apply an {@link OperationOutput} to this output.
 		 *
 		 * @param operationOutput must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		protected abstract B apply(OperationOutput operationOutput);
 
@@ -354,7 +352,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 * Returns the finally to be applied {@link BucketOperation} with the given alias.
 		 *
 		 * @param alias will never be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public T as(String alias) {
 
@@ -550,8 +548,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		public Document toDocument(AggregationOperationContext context) {
 
 			List<Object> operationArguments = getOperationArguments(context);
-			return new Document(operation,
-					operationArguments.size() == 1 ? operationArguments.get(0) : operationArguments);
+			return new Document(operation, operationArguments.size() == 1 ? operationArguments.get(0) : operationArguments);
 		}
 
 		protected List<Object> getOperationArguments(AggregationOperationContext context) {
@@ -579,7 +576,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Returns the field that holds the {@link ProjectionOperationBuilder.OperationProjection}.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		protected Field getField() {
 			return getExposedField();
@@ -589,7 +586,7 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		 * Creates a new instance of this {@link OperationOutput} with the given alias.
 		 *
 		 * @param alias the alias to set
-		 * @return
+		 * @return new instance of {@link OperationOutput}.
 		 */
 		public OperationOutput withAlias(String alias) {
 
@@ -658,8 +655,8 @@ public abstract class BucketOperationSupport<T extends BucketOperationSupport<T,
 		/**
 		 * Creates a new {@link AggregationExpressionOutput}.
 		 *
-		 * @param field
-		 * @param expression
+		 * @param field must not be {@literal null}.
+		 * @param expression must not be {@literal null}.
 		 */
 		protected AggregationExpressionOutput(Field field, AggregationExpression expression) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ComparisonOperators.java

@@ -32,7 +32,7 @@ public class ComparisonOperators {
 	 * Take the field referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ComparisonOperatorFactory}.
 	 */
 	public static ComparisonOperatorFactory valueOf(String fieldReference) {
 		return new ComparisonOperatorFactory(fieldReference);
@@ -42,7 +42,7 @@ public class ComparisonOperators {
 	 * Take the value resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ComparisonOperatorFactory}.
 	 */
 	public static ComparisonOperatorFactory valueOf(AggregationExpression expression) {
 		return new ComparisonOperatorFactory(expression);
@@ -81,7 +81,7 @@ public class ComparisonOperators {
 		 * Creates new {@link AggregationExpression} that compares two values.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareTo(String fieldReference) {
 			return createCmp().compareTo(fieldReference);
@@ -91,7 +91,7 @@ public class ComparisonOperators {
 		 * Creates new {@link AggregationExpression} that compares two values.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareTo(AggregationExpression expression) {
 			return createCmp().compareTo(expression);
@@ -101,7 +101,7 @@ public class ComparisonOperators {
 		 * Creates new {@link AggregationExpression} that compares two values.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareToValue(Object value) {
 			return createCmp().compareToValue(value);
@@ -116,7 +116,7 @@ public class ComparisonOperators {
 		 * value is equal to the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalTo(String fieldReference) {
 			return createEq().equalTo(fieldReference);
@@ -127,7 +127,7 @@ public class ComparisonOperators {
 		 * value is equal to the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalTo(AggregationExpression expression) {
 			return createEq().equalTo(expression);
@@ -138,7 +138,7 @@ public class ComparisonOperators {
 		 * value is equal to the given value.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalToValue(Object value) {
 			return createEq().equalToValue(value);
@@ -153,7 +153,7 @@ public class ComparisonOperators {
 		 * value is greater than the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThan(String fieldReference) {
 			return createGt().greaterThan(fieldReference);
@@ -164,7 +164,7 @@ public class ComparisonOperators {
 		 * value is greater than the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThan(AggregationExpression expression) {
 			return createGt().greaterThan(expression);
@@ -175,7 +175,7 @@ public class ComparisonOperators {
 		 * value is greater than the given value.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThanValue(Object value) {
 			return createGt().greaterThanValue(value);
@@ -190,7 +190,7 @@ public class ComparisonOperators {
 		 * value is greater than or equivalent to the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualTo(String fieldReference) {
 			return createGte().greaterThanEqualTo(fieldReference);
@@ -201,7 +201,7 @@ public class ComparisonOperators {
 		 * value is greater than or equivalent to the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualTo(AggregationExpression expression) {
 			return createGte().greaterThanEqualTo(expression);
@@ -212,7 +212,7 @@ public class ComparisonOperators {
 		 * value is greater than or equivalent to the given value.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualToValue(Object value) {
 			return createGte().greaterThanEqualToValue(value);
@@ -227,7 +227,7 @@ public class ComparisonOperators {
 		 * value is less than the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThan(String fieldReference) {
 			return createLt().lessThan(fieldReference);
@@ -238,7 +238,7 @@ public class ComparisonOperators {
 		 * value is less than the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThan(AggregationExpression expression) {
 			return createLt().lessThan(expression);
@@ -249,7 +249,7 @@ public class ComparisonOperators {
 		 * value is less than to the given value.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThanValue(Object value) {
 			return createLt().lessThanValue(value);
@@ -264,7 +264,7 @@ public class ComparisonOperators {
 		 * value is less than or equivalent to the value of the referenced field.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualTo(String fieldReference) {
 			return createLte().lessThanEqualTo(fieldReference);
@@ -275,7 +275,7 @@ public class ComparisonOperators {
 		 * value is less than or equivalent to the expression result.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualTo(AggregationExpression expression) {
 			return createLte().lessThanEqualTo(expression);
@@ -285,8 +285,8 @@ public class ComparisonOperators {
 		 * Creates new {@link AggregationExpression} that compares two values and returns {@literal true} when the first
 		 * value is less than or equivalent to the given value.
 		 *
-		 * @param value
-		 * @return
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualToValue(Object value) {
 			return createLte().lessThanEqualToValue(value);
@@ -301,7 +301,7 @@ public class ComparisonOperators {
 		 * are not equivalent.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualTo(String fieldReference) {
 			return createNe().notEqualTo(fieldReference);
@@ -312,7 +312,7 @@ public class ComparisonOperators {
 		 * are not equivalent.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualTo(AggregationExpression expression) {
 			return createNe().notEqualTo(expression);
@@ -323,7 +323,7 @@ public class ComparisonOperators {
 		 * are not equivalent.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualToValue(Object value) {
 			return createNe().notEqualToValue(value);
@@ -358,7 +358,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public static Cmp valueOf(String fieldReference) {
 
@@ -370,7 +370,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public static Cmp valueOf(AggregationExpression expression) {
 
@@ -382,7 +382,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareTo(String fieldReference) {
 
@@ -394,7 +394,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareTo(AggregationExpression expression) {
 
@@ -406,12 +406,12 @@ public class ComparisonOperators {
 		 * Creates new {@link Cmp} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Cmp}.
 		 */
 		public Cmp compareToValue(Object value) {
 
 			Assert.notNull(value, "Value must not be null!");
-			return new Cmp(append(value));
+			return new Cmp(append(value, Expand.KEEP_SOURCE));
 		}
 	}
 
@@ -435,7 +435,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public static Eq valueOf(String fieldReference) {
 
@@ -447,7 +447,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public static Eq valueOf(AggregationExpression expression) {
 
@@ -459,7 +459,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalTo(String fieldReference) {
 
@@ -471,7 +471,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalTo(AggregationExpression expression) {
 
@@ -483,12 +483,12 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Eq}.
 		 */
 		public Eq equalToValue(Object value) {
 
 			Assert.notNull(value, "Value must not be null!");
-			return new Eq(append(value));
+			return new Eq(append(value, Expand.KEEP_SOURCE));
 		}
 	}
 
@@ -512,7 +512,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public static Gt valueOf(String fieldReference) {
 
@@ -524,7 +524,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public static Gt valueOf(AggregationExpression expression) {
 
@@ -536,7 +536,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThan(String fieldReference) {
 
@@ -548,7 +548,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThan(AggregationExpression expression) {
 
@@ -560,7 +560,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gt} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gt}.
 		 */
 		public Gt greaterThanValue(Object value) {
 
@@ -589,7 +589,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public static Lt valueOf(String fieldReference) {
 
@@ -601,7 +601,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public static Lt valueOf(AggregationExpression expression) {
 
@@ -613,7 +613,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThan(String fieldReference) {
 
@@ -625,7 +625,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThan(AggregationExpression expression) {
 
@@ -637,7 +637,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lt} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lt}.
 		 */
 		public Lt lessThanValue(Object value) {
 
@@ -666,7 +666,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public static Gte valueOf(String fieldReference) {
 
@@ -678,7 +678,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public static Gte valueOf(AggregationExpression expression) {
 
@@ -690,7 +690,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualTo(String fieldReference) {
 
@@ -702,7 +702,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualTo(AggregationExpression expression) {
 
@@ -714,7 +714,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Gte} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Gte}.
 		 */
 		public Gte greaterThanEqualToValue(Object value) {
 
@@ -743,7 +743,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public static Lte valueOf(String fieldReference) {
 
@@ -755,7 +755,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public static Lte valueOf(AggregationExpression expression) {
 
@@ -767,7 +767,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualTo(String fieldReference) {
 
@@ -779,7 +779,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualTo(AggregationExpression expression) {
 
@@ -791,7 +791,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Lte} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Lte}.
 		 */
 		public Lte lessThanEqualToValue(Object value) {
 
@@ -820,7 +820,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Ne}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public static Ne valueOf(String fieldReference) {
 
@@ -832,7 +832,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Ne}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public static Ne valueOf(AggregationExpression expression) {
 
@@ -844,7 +844,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Ne} with all previously added arguments appending the given one.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualTo(String fieldReference) {
 
@@ -856,7 +856,7 @@ public class ComparisonOperators {
 		 * Creates new {@link Ne} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualTo(AggregationExpression expression) {
 
@@ -868,12 +868,12 @@ public class ComparisonOperators {
 		 * Creates new {@link Eq} with all previously added arguments appending the given one.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Ne}.
 		 */
 		public Ne notEqualToValue(Object value) {
 
 			Assert.notNull(value, "Value must not be null!");
-			return new Ne(append(value));
+			return new Ne(append(value, Expand.KEEP_SOURCE));
 		}
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ConditionalOperators.java

@@ -43,7 +43,7 @@ public class ConditionalOperators {
 	 * Take the field referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ConditionalOperatorFactory}.
 	 */
 	public static ConditionalOperatorFactory when(String fieldReference) {
 		return new ConditionalOperatorFactory(fieldReference);
@@ -53,7 +53,7 @@ public class ConditionalOperators {
 	 * Take the value resulting from the given {@literal expression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ConditionalOperatorFactory}.
 	 */
 	public static ConditionalOperatorFactory when(AggregationExpression expression) {
 		return new ConditionalOperatorFactory(expression);
@@ -63,7 +63,7 @@ public class ConditionalOperators {
 	 * Take the value resulting from the given {@literal criteriaDefinition}.
 	 *
 	 * @param criteriaDefinition must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ConditionalOperatorFactory}.
 	 */
 	public static ConditionalOperatorFactory when(CriteriaDefinition criteriaDefinition) {
 		return new ConditionalOperatorFactory(criteriaDefinition);
@@ -75,7 +75,7 @@ public class ConditionalOperators {
 	 * instances of undefined values or missing fields, returns the value of the replacement expression.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link IfNull.ThenBuilder} to create {@link IfNull}.
 	 */
 	public static IfNull.ThenBuilder ifNull(String fieldReference) {
 
@@ -89,7 +89,7 @@ public class ConditionalOperators {
 	 * instances of undefined values or missing fields, returns the value of the replacement expression.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link IfNull.ThenBuilder} to create {@link IfNull}.
 	 */
 	public static IfNull.ThenBuilder ifNull(AggregationExpression expression) {
 
@@ -103,7 +103,7 @@ public class ConditionalOperators {
 	 * out of the control flow.
 	 *
 	 * @param conditions must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Switch}.
 	 */
 	public static Switch switchCases(CaseOperator... conditions) {
 		return Switch.switchCases(conditions);
@@ -115,7 +115,7 @@ public class ConditionalOperators {
 	 * out of the control flow.
 	 *
 	 * @param conditions must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Switch}.
 	 */
 	public static Switch switchCases(List<CaseOperator> conditions) {
 		return Switch.switchCases(conditions);
@@ -176,7 +176,7 @@ public class ConditionalOperators {
 		 * return expressions.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link OtherwiseBuilder} to create {@link Cond}.
 		 */
 		public OtherwiseBuilder then(Object value) {
 
@@ -189,7 +189,7 @@ public class ConditionalOperators {
 		 * return expressions.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link OtherwiseBuilder} to create {@link Cond}.
 		 */
 		public OtherwiseBuilder thenValueOf(AggregationExpression expression) {
 
@@ -202,7 +202,7 @@ public class ConditionalOperators {
 		 * return expressions.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link OtherwiseBuilder} to create {@link Cond}.
 		 */
 		public OtherwiseBuilder thenValueOf(String fieldReference) {
 
@@ -233,9 +233,9 @@ public class ConditionalOperators {
 	 * field references}, {@link AggregationExpression expressions}, values of simple MongoDB types or values that can be
 	 * converted to a simple MongoDB type.
 	 *
-	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/">https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/</a>
 	 * @author Mark Paluch
+	 * @see <a href=
+	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/">https://docs.mongodb.com/manual/reference/operator/aggregation/ifNull/</a>
 	 */
 	public static class IfNull implements AggregationExpression {
 
@@ -251,9 +251,8 @@ public class ConditionalOperators {
 		/**
 		 * Creates new {@link IfNull}.
 		 *
-		 * @param fieldReference the field to check for a {@literal null} value, field reference must not be {@literal null}
-		 *          .
-		 * @return
+		 * @param fieldReference the field to check for a {@literal null} value, field reference must not be {@literal null}.
+		 * @return never {@literal null}.
 		 */
 		public static ThenBuilder ifNull(String fieldReference) {
 
@@ -265,8 +264,8 @@ public class ConditionalOperators {
 		 * Creates new {@link IfNull}.
 		 *
 		 * @param expression the expression to check for a {@literal null} value, field reference must not be
-		 *          {@literal null}.
-		 * @return
+		 * {@literal null}.
+		 * @return never {@literal null}.
 		 */
 		public static ThenBuilder ifNull(AggregationExpression expression) {
 
@@ -316,14 +315,14 @@ public class ConditionalOperators {
 
 			/**
 			 * @param fieldReference the field to check for a {@literal null} value, field reference must not be
-			 *          {@literal null}.
+			 * {@literal null}.
 			 * @return the {@link ThenBuilder}
 			 */
 			ThenBuilder ifNull(String fieldReference);
 
 			/**
 			 * @param expression the expression to check for a {@literal null} value, field name must not be {@literal null}
-			 *          or empty.
+			 * or empty.
 			 * @return the {@link ThenBuilder}
 			 */
 			ThenBuilder ifNull(AggregationExpression expression);
@@ -336,21 +335,21 @@ public class ConditionalOperators {
 
 			/**
 			 * @param value the value to be used if the {@code $ifNull} condition evaluates {@literal true}. Can be a
-			 *          {@link Document}, a value that is supported by MongoDB or a value that can be converted to a MongoDB
-			 *          representation but must not be {@literal null}.
-			 * @return
+			 * {@link Document}, a value that is supported by MongoDB or a value that can be converted to a MongoDB
+			 * representation but must not be {@literal null}.
+			 * @return new instance of {@link IfNull}.
 			 */
 			IfNull then(Object value);
 
 			/**
 			 * @param fieldReference the field holding the replacement value, must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IfNull}.
 			 */
 			IfNull thenValueOf(String fieldReference);
 
 			/**
 			 * @param expression the expression yielding to the replacement value, must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IfNull}.
 			 */
 			IfNull thenValueOf(AggregationExpression expression);
 		}
@@ -364,7 +363,8 @@ public class ConditionalOperators {
 
 			private @Nullable Object condition;
 
-			private IfNullOperatorBuilder() {}
+			private IfNullOperatorBuilder() {
+			}
 
 			/**
 			 * Creates a new builder for {@link IfNull}.
@@ -458,9 +458,15 @@ public class ConditionalOperators {
 		public static Switch switchCases(List<CaseOperator> conditions) {
 
 			Assert.notNull(conditions, "Conditions must not be null!");
-			return new Switch(Collections.<String, Object> singletonMap("branches", new ArrayList<CaseOperator>(conditions)));
+			return new Switch(Collections.<String, Object>singletonMap("branches", new ArrayList<CaseOperator>(conditions)));
 		}
 
+		/**
+		 * Set the default value.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Switch}.
+		 */
 		public Switch defaultTo(Object value) {
 			return new Switch(append("default", value));
 		}
@@ -522,7 +528,7 @@ public class ConditionalOperators {
 				 * Set the then {@literal value}.
 				 *
 				 * @param value must not be {@literal null}.
-				 * @return
+				 * @return new instance of {@link CaseOperator}.
 				 */
 				CaseOperator then(Object value);
 			}
@@ -536,10 +542,10 @@ public class ConditionalOperators {
 	 * {@link AggregationExpression expressions}, values of simple MongoDB types or values that can be converted to a
 	 * simple MongoDB type.
 	 *
-	 * @see <a href=
-	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/cond/">https://docs.mongodb.com/manual/reference/operator/aggregation/cond/</a>
 	 * @author Mark Paluch
 	 * @author Christoph Strobl
+	 * @see <a href=
+	 * "https://docs.mongodb.com/manual/reference/operator/aggregation/cond/">https://docs.mongodb.com/manual/reference/operator/aggregation/cond/</a>
 	 */
 	public static class Cond implements AggregationExpression {
 
@@ -664,7 +670,6 @@ public class ConditionalOperators {
 				}
 
 				clauses.add(new Document(key, args));
-
 			} else if (predicate instanceof Document) {
 
 				Document nested = (Document) predicate;
@@ -680,7 +685,6 @@ public class ConditionalOperators {
 					args.add(nested.get(s));
 					clauses.add(new Document(s, args));
 				}
-
 			} else if (!isKeyword(key)) {
 
 				List<Object> args = new ArrayList<Object>();
@@ -802,8 +806,8 @@ public class ConditionalOperators {
 
 			/**
 			 * @param value the value to be used if the condition evaluates {@literal true}. Can be a {@link Document}, a
-			 *          value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
-			 *          must not be {@literal null}.
+			 * value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
+			 * must not be {@literal null}.
 			 * @return the {@link OtherwiseBuilder}
 			 */
 			OtherwiseBuilder then(Object value);
@@ -828,8 +832,8 @@ public class ConditionalOperators {
 
 			/**
 			 * @param value the value to be used if the condition evaluates {@literal false}. Can be a {@link Document}, a
-			 *          value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
-			 *          must not be {@literal null}.
+			 * value that is supported by MongoDB or a value that can be converted to a MongoDB representation but
+			 * must not be {@literal null}.
 			 * @return the {@link Cond}
 			 */
 			Cond otherwise(Object value);
@@ -857,7 +861,8 @@ public class ConditionalOperators {
 			private @Nullable Object condition;
 			private @Nullable Object thenValue;
 
-			private ConditionalExpressionBuilder() {}
+			private ConditionalExpressionBuilder() {
+			}
 
 			/**
 			 * Creates a new builder for {@link Cond}.

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/CountOperation.java

@@ -34,9 +34,9 @@ public class CountOperation implements FieldsExposingAggregationOperation {
 	private final String fieldName;
 
 	/**
-	 * Creates a new {@link CountOperation} given the {@link fieldName} field name.
+	 * Creates a new {@link CountOperation} given the {@literal fieldName} field name.
 	 *
-	 * @param asFieldName must not be {@literal null} or empty.
+	 * @param fieldName must not be {@literal null} or empty.
 	 */
 	public CountOperation(String fieldName) {
 
@@ -49,7 +49,12 @@ public class CountOperation implements FieldsExposingAggregationOperation {
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document("$count", fieldName);
+		return new Document(getOperator(), fieldName);
+	}
+
+	@Override
+	public String getOperator() {
+		return "$count";
 	}
 
 	/* (non-Javadoc)
@@ -71,7 +76,7 @@ public class CountOperation implements FieldsExposingAggregationOperation {
 		 * Returns the finally to be applied {@link CountOperation} with the given alias.
 		 *
 		 * @param fieldName must not be {@literal null} or empty.
-		 * @return
+		 * @return new instance of {@link CountOperation}.
 		 */
 		public CountOperation as(String fieldName) {
 			return new CountOperation(fieldName);

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/DataTypeOperators.java

@@ -30,7 +30,7 @@ public class DataTypeOperators {
 	 * Return the BSON data type of the given {@literal field}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link Type}.
 	 */
 	public static Type typeOf(String fieldReference) {
 		return Type.typeOf(fieldReference);
@@ -56,7 +56,7 @@ public class DataTypeOperators {
 		 * Creates new {@link Type}.
 		 *
 		 * @param field must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Type}.
 		 */
 		public static Type typeOf(String field) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/DateOperators.java

@@ -37,7 +37,7 @@ public class DateOperators {
 	 * Take the date referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link DateOperatorFactory}.
 	 */
 	public static DateOperatorFactory dateOf(String fieldReference) {
 
@@ -49,7 +49,7 @@ public class DateOperators {
 	 * Take the date resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link DateOperatorFactory}.
 	 */
 	public static DateOperatorFactory dateOf(AggregationExpression expression) {
 
@@ -278,7 +278,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the day of the year for a date as a number between 1 and
 		 * 366.
 		 *
-		 * @return
+		 * @return new instance of {@link DayOfYear}.
 		 */
 		public DayOfYear dayOfYear() {
 			return applyTimezone(DayOfYear.dayOfYear(dateReference()), timezone);
@@ -288,7 +288,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the day of the month for a date as a number between 1 and
 		 * 31.
 		 *
-		 * @return
+		 * @return new instance of {@link DayOfMonth}.
 		 */
 		public DayOfMonth dayOfMonth() {
 			return applyTimezone(DayOfMonth.dayOfMonth(dateReference()), timezone);
@@ -298,7 +298,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the day of the week for a date as a number between 1
 		 * (Sunday) and 7 (Saturday).
 		 *
-		 * @return
+		 * @return new instance of {@link DayOfWeek}.
 		 */
 		public DayOfWeek dayOfWeek() {
 			return applyTimezone(DayOfWeek.dayOfWeek(dateReference()), timezone);
@@ -307,7 +307,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the year portion of a date.
 		 *
-		 * @return
+		 * @return new instance of {@link Year}.
 		 */
 		public Year year() {
 			return applyTimezone(Year.year(dateReference()), timezone);
@@ -316,7 +316,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the month of a date as a number between 1 and 12.
 		 *
-		 * @return
+		 * @return new instance of {@link Month}.
 		 */
 		public Month month() {
 			return applyTimezone(Month.month(dateReference()), timezone);
@@ -326,7 +326,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the week of the year for a date as a number between 0 and
 		 * 53.
 		 *
-		 * @return
+		 * @return new instance of {@link Week}.
 		 */
 		public Week week() {
 			return applyTimezone(Week.week(dateReference()), timezone);
@@ -335,7 +335,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the hour portion of a date as a number between 0 and 23.
 		 *
-		 * @return
+		 * @return new instance of {@link Hour}.
 		 */
 		public Hour hour() {
 			return applyTimezone(Hour.hour(dateReference()), timezone);
@@ -344,7 +344,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the minute portion of a date as a number between 0 and 59.
 		 *
-		 * @return
+		 * @return new instance of {@link Minute}.
 		 */
 		public Minute minute() {
 			return applyTimezone(Minute.minute(dateReference()), timezone);
@@ -354,7 +354,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the second portion of a date as a number between 0 and 59,
 		 * but can be 60 to account for leap seconds.
 		 *
-		 * @return
+		 * @return new instance of {@link Second}.
 		 */
 		public Second second() {
 			return applyTimezone(Second.second(dateReference()), timezone);
@@ -364,7 +364,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the millisecond portion of a date as an integer between 0
 		 * and 999.
 		 *
-		 * @return
+		 * @return new instance of {@link Millisecond}.
 		 */
 		public Millisecond millisecond() {
 			return applyTimezone(Millisecond.millisecond(dateReference()), timezone);
@@ -375,7 +375,7 @@ public class DateOperators {
 		 * {@literal format}.
 		 *
 		 * @param format must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DateToString}.
 		 */
 		public DateToString toString(String format) {
 			return applyTimezone(DateToString.dateToString(dateReference()).toString(format), timezone);
@@ -396,7 +396,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the weekday number in ISO 8601-2018 format, ranging from 1
 		 * (for Monday) to 7 (for Sunday).
 		 *
-		 * @return
+		 * @return new instance of {@link IsoDayOfWeek}.
 		 */
 		public IsoDayOfWeek isoDayOfWeek() {
 			return applyTimezone(IsoDayOfWeek.isoDayWeek(dateReference()), timezone);
@@ -406,7 +406,7 @@ public class DateOperators {
 		 * Creates new {@link AggregationExpression} that returns the week number in ISO 8601-2018 format, ranging from 1 to
 		 * 53.
 		 *
-		 * @return
+		 * @return new instance of {@link IsoWeek}.
 		 */
 		public IsoWeek isoWeek() {
 			return applyTimezone(IsoWeek.isoWeek(dateReference()), timezone);
@@ -415,7 +415,7 @@ public class DateOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that returns the year number in ISO 8601-2018 format.
 		 *
-		 * @return
+		 * @return new instance of {@link IsoWeekYear}.
 		 */
 		public IsoWeekYear isoWeekYear() {
 			return applyTimezone(IsoWeekYear.isoWeekYear(dateReference()), timezone);
@@ -642,7 +642,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfYear}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfYear}.
 		 */
 		public static DayOfYear dayOfYear(String fieldReference) {
 
@@ -654,7 +654,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfYear}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfYear}.
 		 */
 		public static DayOfYear dayOfYear(AggregationExpression expression) {
 
@@ -714,7 +714,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfMonth}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfMonth}.
 		 */
 		public static DayOfMonth dayOfMonth(String fieldReference) {
 
@@ -726,7 +726,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfMonth}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfMonth}.
 		 */
 		public static DayOfMonth dayOfMonth(AggregationExpression expression) {
 
@@ -786,7 +786,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfWeek}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfWeek}.
 		 */
 		public static DayOfWeek dayOfWeek(String fieldReference) {
 
@@ -798,7 +798,7 @@ public class DateOperators {
 		 * Creates new {@link DayOfWeek}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link DayOfWeek}.
 		 */
 		public static DayOfWeek dayOfWeek(AggregationExpression expression) {
 
@@ -858,7 +858,7 @@ public class DateOperators {
 		 * Creates new {@link Year}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Year}.
 		 */
 		public static Year yearOf(String fieldReference) {
 
@@ -870,7 +870,7 @@ public class DateOperators {
 		 * Creates new {@link Year}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Year}.
 		 */
 		public static Year yearOf(AggregationExpression expression) {
 
@@ -930,7 +930,7 @@ public class DateOperators {
 		 * Creates new {@link Month}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Month}.
 		 */
 		public static Month monthOf(String fieldReference) {
 
@@ -942,7 +942,7 @@ public class DateOperators {
 		 * Creates new {@link Month}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Month}.
 		 */
 		public static Month monthOf(AggregationExpression expression) {
 
@@ -1002,7 +1002,7 @@ public class DateOperators {
 		 * Creates new {@link Week}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Week}.
 		 */
 		public static Week weekOf(String fieldReference) {
 
@@ -1014,7 +1014,7 @@ public class DateOperators {
 		 * Creates new {@link Week}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Week}.
 		 */
 		public static Week weekOf(AggregationExpression expression) {
 
@@ -1074,7 +1074,7 @@ public class DateOperators {
 		 * Creates new {@link Hour}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Hour}.
 		 */
 		public static Hour hourOf(String fieldReference) {
 
@@ -1086,7 +1086,7 @@ public class DateOperators {
 		 * Creates new {@link Hour}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Hour}.
 		 */
 		public static Hour hourOf(AggregationExpression expression) {
 
@@ -1146,7 +1146,7 @@ public class DateOperators {
 		 * Creates new {@link Minute}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Minute}.
 		 */
 		public static Minute minuteOf(String fieldReference) {
 
@@ -1158,7 +1158,7 @@ public class DateOperators {
 		 * Creates new {@link Minute}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Minute}.
 		 */
 		public static Minute minuteOf(AggregationExpression expression) {
 
@@ -1218,7 +1218,7 @@ public class DateOperators {
 		 * Creates new {@link Second}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Second}.
 		 */
 		public static Second secondOf(String fieldReference) {
 
@@ -1230,7 +1230,7 @@ public class DateOperators {
 		 * Creates new {@link Second}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Second}.
 		 */
 		public static Second secondOf(AggregationExpression expression) {
 
@@ -1290,7 +1290,7 @@ public class DateOperators {
 		 * Creates new {@link Millisecond}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Millisecond}.
 		 */
 		public static Millisecond millisecondOf(String fieldReference) {
 
@@ -1302,7 +1302,7 @@ public class DateOperators {
 		 * Creates new {@link Millisecond}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Millisecond}.
 		 */
 		public static Millisecond millisecondOf(AggregationExpression expression) {
 
@@ -1376,7 +1376,7 @@ public class DateOperators {
 		 * Creates new {@link FormatBuilder} allowing to define the date format to apply.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link FormatBuilder} to crate {@link DateToString}.
 		 */
 		public static FormatBuilder dateOf(final String fieldReference) {
 
@@ -1388,7 +1388,7 @@ public class DateOperators {
 		 * Creates new {@link FormatBuilder} allowing to define the date format to apply.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link FormatBuilder} to crate {@link DateToString}.
 		 */
 		public static FormatBuilder dateOf(final AggregationExpression expression) {
 
@@ -1540,7 +1540,7 @@ public class DateOperators {
 		 * Creates new {@link IsoDayOfWeek}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoDayOfWeek}.
 		 */
 		public static IsoDayOfWeek isoDayOfWeek(String fieldReference) {
 
@@ -1552,7 +1552,7 @@ public class DateOperators {
 		 * Creates new {@link IsoDayOfWeek}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoDayOfWeek}.
 		 */
 		public static IsoDayOfWeek isoDayOfWeek(AggregationExpression expression) {
 
@@ -1612,7 +1612,7 @@ public class DateOperators {
 		 * Creates new {@link IsoWeek}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoWeek}.
 		 */
 		public static IsoWeek isoWeekOf(String fieldReference) {
 
@@ -1624,7 +1624,7 @@ public class DateOperators {
 		 * Creates new {@link IsoWeek}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoWeek}.
 		 */
 		public static IsoWeek isoWeekOf(AggregationExpression expression) {
 
@@ -1684,7 +1684,7 @@ public class DateOperators {
 		 * Creates new {@link IsoWeekYear}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoWeekYear}.
 		 */
 		public static IsoWeekYear isoWeekYearOf(String fieldReference) {
 
@@ -1696,7 +1696,7 @@ public class DateOperators {
 		 * Creates new {@link Millisecond}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IsoWeekYear}.
 		 */
 		public static IsoWeekYear isoWeekYearOf(AggregationExpression expression) {
 
@@ -2378,6 +2378,7 @@ public class DateOperators {
 	@SuppressWarnings("unchecked")
 	private static <T extends TimezonedDateAggregationExpression> T applyTimezone(T instance, Timezone timezone) {
 		return !ObjectUtils.nullSafeEquals(Timezone.none(), timezone) && !instance.hasTimezone()
-				? (T) instance.withTimezone(timezone) : instance;
+				? (T) instance.withTimezone(timezone)
+				: instance;
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/DocumentEnhancingOperation.java

@@ -71,6 +71,15 @@ abstract class DocumentEnhancingOperation implements InheritsFieldsAggregationOp
 	 */
 	protected abstract String mongoOperator();
 
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return mongoOperator();
+	}
+
 	/**
 	 * @return the raw value map
 	 */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ExposedFields.java

@@ -46,7 +46,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	/**
 	 * Returns an empty {@link ExposedFields} instance.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 * @since 2.0
 	 */
 	public static ExposedFields empty() {
@@ -57,7 +57,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates a new {@link ExposedFields} instance from the given {@link ExposedField}s.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static ExposedFields from(ExposedField... fields) {
 		return from(Arrays.asList(fields));
@@ -67,7 +67,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates a new {@link ExposedFields} instance from the given {@link ExposedField}s.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	private static ExposedFields from(List<ExposedField> fields) {
 
@@ -84,7 +84,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates synthetic {@link ExposedFields} from the given {@link Fields}.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static ExposedFields synthetic(Fields fields) {
 		return createFields(fields, true);
@@ -94,7 +94,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates non-synthetic {@link ExposedFields} from the given {@link Fields}.
 	 *
 	 * @param fields must not be {@literal null}.
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static ExposedFields nonSynthetic(Fields fields) {
 		return createFields(fields, false);
@@ -105,7 +105,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 *
 	 * @param fields must not be {@literal null}.
 	 * @param synthetic
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	private static ExposedFields createFields(Fields fields, boolean synthetic) {
 
@@ -135,7 +135,7 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	 * Creates a new {@link ExposedFields} adding the given {@link ExposedField}.
 	 *
 	 * @param field must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ExposedFields}.
 	 */
 	public ExposedFields and(ExposedField field) {
 
@@ -151,8 +151,8 @@ public final class ExposedFields implements Iterable<ExposedField> {
 	/**
 	 * Returns the field with the given name or {@literal null} if no field with the given name is available.
 	 *
-	 * @param name
-	 * @return
+	 * @param name must not be {@literal null}.
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	public ExposedField getField(String name) {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/FacetOperation.java

@@ -20,12 +20,11 @@ import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
 
+import org.bson.Document;
 import org.springframework.data.mongodb.core.aggregation.BucketOperationSupport.Output;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.util.Assert;
 
-import org.bson.Document;
-
 /**
  * Encapsulates the aggregation framework {@code $facet}-operation. <br />
  * Facet of {@link AggregationOperation}s to be used in an {@link Aggregation}. Processes multiple
@@ -84,7 +83,16 @@ public class FacetOperation implements FieldsExposingAggregationOperation {
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document("$facet", facets.toDocument(context));
+		return new Document(getOperator(), facets.toDocument(context));
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$facet";
 	}
 
 	/* (non-Javadoc)

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GeoNearOperation.java

@@ -16,14 +16,12 @@
 package org.springframework.data.mongodb.core.aggregation;
 
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 
 import org.bson.Document;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
-import org.springframework.util.NumberUtils;
 import org.springframework.util.StringUtils;
 
 /**
@@ -95,21 +93,27 @@ public class GeoNearOperation implements AggregationOperation {
 
 		Document command = context.getMappedObject(nearQuery.toDocument());
 
-		if(command.containsKey("query")) {
+		if (command.containsKey("query")) {
 			command.replace("query", context.getMappedObject(command.get("query", Document.class)));
 		}
 
-		if(command.containsKey("collation")) {
-			command.remove("collation");
-		}
-
+		command.remove("collation");
 		command.put("distanceField", distanceField);
 
 		if (StringUtils.hasText(indexKey)) {
 			command.put("key", indexKey);
 		}
 
-		return new Document("$geoNear", command);
+		return new Document(getOperator(), command);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$geoNear";
 	}
 
 	/*
@@ -125,11 +129,11 @@ public class GeoNearOperation implements AggregationOperation {
 		List<Document> stages = new ArrayList<>();
 		stages.add(command);
 
-		if(nearQuery.getSkip() != null && nearQuery.getSkip() > 0){
+		if (nearQuery.getSkip() != null && nearQuery.getSkip() > 0) {
 			stages.add(new Document("$skip", nearQuery.getSkip()));
 		}
 
-		if(limit != null) {
+		if (limit != null) {
 			stages.add(new Document("$limit", limit.longValue()));
 		}
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GraphLookupOperation.java

@@ -119,7 +119,12 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 			graphLookup.put("restrictSearchWithMatch", context.getMappedObject(restrictSearchWithMatch.getCriteriaObject()));
 		}
 
-		return new Document("$graphLookup", graphLookup);
+		return new Document(getOperator(), graphLookup);
+	}
+
+	@Override
+	public String getOperator() {
+		return "$graphLookup";
 	}
 
 	/*
@@ -146,7 +151,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the {@literal collectionName} to apply the {@code $graphLookup} to.
 		 *
 		 * @param collectionName must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		StartWithBuilder from(String collectionName);
 	}
@@ -161,7 +166,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the startWith {@literal fieldReferences} to apply the {@code $graphLookup} to.
 		 *
 		 * @param fieldReferences must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		ConnectFromBuilder startWith(String... fieldReferences);
 
@@ -169,7 +174,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the startWith {@literal expressions} to apply the {@code $graphLookup} to.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		ConnectFromBuilder startWith(AggregationExpression... expressions);
 
@@ -178,7 +183,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * {@link AggregationExpression} to apply the {@code $graphLookup} to.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 * @throws IllegalArgumentException
 		 */
 		ConnectFromBuilder startWith(Object... expressions);
@@ -193,7 +198,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the connectFrom {@literal fieldName} to apply the {@code $graphLookup} to.
 		 *
 		 * @param fieldName must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		ConnectToBuilder connectFrom(String fieldName);
 	}
@@ -207,7 +212,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Set the connectTo {@literal fieldName} to apply the {@code $graphLookup} to.
 		 *
 		 * @param fieldName must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		GraphLookupOperationBuilder connectTo(String fieldName);
 	}
@@ -360,7 +365,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Optionally limit the number of recursions.
 		 *
 		 * @param numberOfRecursions must be greater or equal to zero.
-		 * @return
+		 * @return this.
 		 */
 		public GraphLookupOperationBuilder maxDepth(long numberOfRecursions) {
 
@@ -374,7 +379,7 @@ public class GraphLookupOperation implements InheritsFieldsAggregationOperation
 		 * Optionally add a depth field {@literal fieldName} to each traversed document in the search path.
 		 *
 		 * @param fieldName must not be {@literal null} or empty.
-		 * @return
+		 * @return this.
 		 */
 		public GraphLookupOperationBuilder depthField(String fieldName) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/GroupOperation.java

@@ -429,7 +429,16 @@ public class GroupOperation implements FieldsExposingAggregationOperation {
 			operationObject.putAll(operation.toDocument(context));
 		}
 
-		return new Document("$group", operationObject);
+		return new Document(getOperator(), operationObject);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$group";
 	}
 
 	interface Keyword {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/LimitOperation.java

@@ -49,6 +49,15 @@ public class LimitOperation implements AggregationOperation {
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document("$limit", Long.valueOf(maxElements));
+		return new Document(getOperator(), Long.valueOf(maxElements));
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$limit";
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/LiteralOperators.java

@@ -76,6 +76,10 @@ public class LiteralOperators {
 			super(value);
 		}
 
+		/* 
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AbstractAggregationExpression#getMongoMethod()
+		 */
 		@Override
 		protected String getMongoMethod() {
 			return "$literal";

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/LookupOperation.java

@@ -83,13 +83,22 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 		lookupObject.append("foreignField", foreignField.getTarget());
 		lookupObject.append("as", as.getTarget());
 
-		return new Document("$lookup", lookupObject);
+		return new Document(getOperator(), lookupObject);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$lookup";
 	}
 
 	/**
 	 * Get a builder that allows creation of {@link LookupOperation}.
 	 *
-	 * @return
+	 * @return never {@literal null}.
 	 */
 	public static FromBuilder newLookup() {
 		return new LookupOperationBuilder();
@@ -99,7 +108,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 
 		/**
 		 * @param name the collection in the same database to perform the join with, must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		LocalFieldBuilder from(String name);
 	}
@@ -109,7 +118,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 		/**
 		 * @param name the field from the documents input to the {@code $lookup} stage, must not be {@literal null} or
 		 *          empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		ForeignFieldBuilder localField(String name);
 	}
@@ -118,7 +127,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 
 		/**
 		 * @param name the field from the documents in the {@code from} collection, must not be {@literal null} or empty.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		AsBuilder foreignField(String name);
 	}
@@ -127,7 +136,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 
 		/**
 		 * @param name the name of the new array field to add to the input documents, must not be {@literal null} or empty.
-		 * @return
+		 * @return new instance of {@link LookupOperation}.
 		 */
 		LookupOperation as(String name);
 	}
@@ -168,8 +177,7 @@ public class LookupOperation implements FieldsExposingAggregationOperation, Inhe
 
 			Assert.hasText(name, "'As' must not be null or empty!");
 			as = new ExposedField(Fields.field(name), true);
-			return new LookupOperation(from, localField, foreignField,
-					as);
+			return new LookupOperation(from, localField, foreignField, as);
 		}
 
 		@Override

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/MatchOperation.java

@@ -30,7 +30,8 @@ import org.springframework.util.Assert;
  * @author Thomas Darimont
  * @author Oliver Gierke
  * @since 1.3
- * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/match/">MongoDB Aggregation Framework: $match</a>
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/match/">MongoDB Aggregation Framework:
+ *      $match</a>
  */
 public class MatchOperation implements AggregationOperation {
 
@@ -53,6 +54,15 @@ public class MatchOperation implements AggregationOperation {
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document("$match", context.getMappedObject(criteriaDefinition.getCriteriaObject()));
+		return new Document(getOperator(), context.getMappedObject(criteriaDefinition.getCriteriaObject()));
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$match";
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/MergeOperation.java

@@ -100,7 +100,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 	public Document toDocument(AggregationOperationContext context) {
 
 		if (isJustCollection()) {
-			return new Document("$merge", into.collection);
+			return new Document(getOperator(), into.collection);
 		}
 
 		Document $merge = new Document();
@@ -122,7 +122,16 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 			$merge.putAll(whenNotMatched.toDocument(context));
 		}
 
-		return new Document("$merge", $merge);
+		return new Document(getOperator(), $merge);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$merge";
 	}
 
 	/*
@@ -186,7 +195,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 		/**
 		 * Merge Documents by using the MongoDB {@literal _id} field.
 		 *
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		public static UniqueMergeId id() {
 			return ID;
@@ -375,7 +384,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 		/**
 		 * Factory method creating {@link WhenDocumentsDontMatch} from a {@code value} literal.
 		 *
-		 * @param value
+		 * @param value must not be {@literal null}.
 		 * @return new instance of {@link WhenDocumentsDontMatch}.
 		 */
 		public static WhenDocumentsDontMatch whenNotMatchedOf(String value) {
@@ -403,7 +412,7 @@ public class MergeOperation implements FieldsExposingAggregationOperation, Inher
 		/**
 		 * Stop and fail the aggregation operation. Does not revert already performed changes on previous documents.
 		 *
-		 * @return
+		 * @return new instance of {@link WhenDocumentsDontMatch}.
 		 */
 		public static WhenDocumentsDontMatch failWhenNotMatch() {
 			return whenNotMatchedOf("fail");

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/OutOperation.java

@@ -16,7 +16,6 @@
 package org.springframework.data.mongodb.core.aggregation;
 
 import org.bson.Document;
-
 import org.springframework.data.mongodb.util.BsonUtils;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
@@ -211,7 +210,16 @@ public class OutOperation implements AggregationOperation {
 			$out.append("uniqueKey", uniqueKey);
 		}
 
-		return new Document("$out", $out);
+		return new Document(getOperator(), $out);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$out";
 	}
 
 	private boolean requiresMongoDb42Format() {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/PrefixingDelegatingAggregationOperationContext.java

@@ -31,7 +31,7 @@ import org.springframework.lang.Nullable;
  * {@link AggregationOperationContext} implementation prefixing non-command keys on root level with the given prefix.
  * Useful when mapping fields to domain specific types while having to prefix keys for query purpose.
  * <p />
- * Fields to be excluded from prefixing my be added to a {@literal blacklist}.
+ * Fields to be excluded from prefixing my be added to a {@literal denylist}.
  *
  * @author Christoph Strobl
  * @author Mark Paluch
@@ -41,18 +41,18 @@ public class PrefixingDelegatingAggregationOperationContext implements Aggregati
 
 	private final AggregationOperationContext delegate;
 	private final String prefix;
-	private final Set<String> blacklist;
+	private final Set<String> denylist;
 
 	public PrefixingDelegatingAggregationOperationContext(AggregationOperationContext delegate, String prefix) {
 		this(delegate, prefix, Collections.emptySet());
 	}
 
 	public PrefixingDelegatingAggregationOperationContext(AggregationOperationContext delegate, String prefix,
-			Collection<String> blacklist) {
+			Collection<String> denylist) {
 
 		this.delegate = delegate;
 		this.prefix = prefix;
-		this.blacklist = new HashSet<>(blacklist);
+		this.denylist = new HashSet<>(denylist);
 	}
 
 	/*
@@ -121,7 +121,7 @@ public class PrefixingDelegatingAggregationOperationContext implements Aggregati
 	}
 
 	private String prefixKey(String key) {
-		return (key.startsWith("$") || isBlacklisted(key)) ? key : (prefix + "." + key);
+		return (key.startsWith("$") || isDenied(key)) ? key : (prefix + "." + key);
 	}
 
 	private Object prefixCollection(Collection<Object> sourceCollection) {
@@ -139,9 +139,9 @@ public class PrefixingDelegatingAggregationOperationContext implements Aggregati
 		return prefixed;
 	}
 
-	private boolean isBlacklisted(String key) {
+	private boolean isDenied(String key) {
 
-		if (blacklist.contains(key)) {
+		if (denylist.contains(key)) {
 			return true;
 		}
 
@@ -149,8 +149,8 @@ public class PrefixingDelegatingAggregationOperationContext implements Aggregati
 			return false;
 		}
 
-		for (String blacklisted : blacklist) {
-			if (key.startsWith(blacklisted + ".")) {
+		for (String denied : denylist) {
+			if (key.startsWith(denied + ".")) {
 				return true;
 			}
 		}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ProjectionOperation.java

@@ -261,7 +261,16 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			fieldObject.putAll(projection.toDocument(context));
 		}
 
-		return new Document("$project", fieldObject);
+		return new Document(getOperator(), fieldObject);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$project";
 	}
 
 	/**
@@ -1548,7 +1557,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 				final Field aliasedField = Fields.field(alias, this.field.getName());
 				return new OperationProjection(aliasedField, operation, values.toArray()) {
 
-					/* 
+					/*
 					 * (non-Javadoc)
 					 * @see org.springframework.data.mongodb.core.aggregation.ProjectionOperation.ProjectionOperationBuilder.OperationProjection#getField()
 					 */
@@ -1749,7 +1758,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			this.expression = expression;
 		}
 
-		/* 
+		/*
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.core.aggregation.ProjectionOperation.Projection#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
 		 */
@@ -1877,7 +1886,7 @@ public class ProjectionOperation implements FieldsExposingAggregationOperation {
 			this.projections = projections;
 		}
 
-		/* 
+		/*
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.core.aggregation.ProjectionOperation.Projection#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
 		 */

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/RedactOperation.java

@@ -74,7 +74,16 @@ public class RedactOperation implements AggregationOperation {
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document("$redact", condition.toDocument(context));
+		return new Document(getOperator(), condition.toDocument(context));
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$redact";
 	}
 
 	/**

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ReplaceRootOperation.java

@@ -86,6 +86,15 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		return new Document("$replaceRoot", new Document("newRoot", getReplacement().toDocumentExpression(context)));
 	}
 
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$replaceRoot";
+	}
+
 	/* (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation#getFields()
 	 */
@@ -166,7 +175,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 	 *
 	 * @author Mark Paluch
 	 */
-	static class ReplaceRootDocumentOperation extends ReplaceRootOperation {
+	public static class ReplaceRootDocumentOperation extends ReplaceRootOperation {
 
 		private final static ReplacementDocument EMPTY = new ReplacementDocument();
 		private final ReplacementDocument current;
@@ -346,7 +355,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		/**
 		 * Creates a {@link ReplacementDocument} given {@link ReplacementContributor}.
 		 *
-		 * @param contributor
+		 * @param contributor must not be {@literal null}.
 		 */
 		protected ReplacementDocument(ReplacementContributor contributor) {
 
@@ -362,7 +371,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		 * Creates a {@link ReplacementDocument} given a {@literal value}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReplacementDocument}.
 		 */
 		public static ReplacementDocument valueOf(Object value) {
 			return new ReplacementDocument(new DocumentContributor(value));
@@ -372,7 +381,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		 * Creates a {@link ReplacementDocument} given a single {@literal field} and {@link AggregationExpression}.
 		 *
 		 * @param aggregationExpression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReplacementDocument}.
 		 */
 		public static ReplacementDocument forExpression(String field, AggregationExpression aggregationExpression) {
 			return new ReplacementDocument(new ExpressionFieldContributor(Fields.field(field), aggregationExpression));
@@ -382,7 +391,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		 * Creates a {@link ReplacementDocument} given a single {@literal field} and {@literal value}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link ReplacementDocument}.
 		 */
 		public static ReplacementDocument forSingleValue(String field, Object value) {
 			return new ReplacementDocument(new ValueFieldContributor(Fields.field(field), value));
@@ -438,7 +447,7 @@ public class ReplaceRootOperation implements FieldsExposingAggregationOperation
 		 * {@link AggregationOperationContext}.
 		 *
 		 * @param context will never be {@literal null}.
-		 * @return
+		 * @return never {@literal null}.
 		 */
 		Document toDocument(AggregationOperationContext context);
 	}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ReplaceWithOperation.java

@@ -57,7 +57,7 @@ public class ReplaceWithOperation extends ReplaceRootOperation {
 	 * {@link Field field reference}.
 	 *
 	 * @param value must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link ReplaceWithOperation}.
 	 */
 	public static ReplaceWithOperation replaceWithValueOf(Object value) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SampleOperation.java

@@ -48,6 +48,15 @@ public class SampleOperation implements AggregationOperation {
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document("$sample", new Document("size", this.sampleSize));
+		return new Document(getOperator(), new Document("size", this.sampleSize));
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$sample";
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SetOperation.java

@@ -99,6 +99,10 @@ public class SetOperation extends DocumentEnhancingOperation {
 		return new FieldAppender(getValueMap());
 	}
 
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.DocumentEnhancingOperation#mongoOperator()
+	 */
 	@Override
 	protected String mongoOperator() {
 		return "$set";

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SetOperators.java

@@ -34,7 +34,7 @@ public class SetOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SetOperatorFactory}.
 	 */
 	public static SetOperatorFactory arrayAsSet(String fieldReference) {
 		return new SetOperatorFactory(fieldReference);
@@ -44,7 +44,7 @@ public class SetOperators {
 	 * Take the array resulting from the given {@link AggregationExpression}.
 	 *
 	 * @param expression must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link SetOperatorFactory}.
 	 */
 	public static SetOperatorFactory arrayAsSet(AggregationExpression expression) {
 		return new SetOperatorFactory(expression);
@@ -87,7 +87,7 @@ public class SetOperators {
 		 * returns {@literal true} if they have the same distinct elements and {@literal false} otherwise.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(String... arrayReferences) {
 			return createSetEquals().isEqualTo(arrayReferences);
@@ -98,7 +98,7 @@ public class SetOperators {
 		 * returns {@literal true} if they have the same distinct elements and {@literal false} otherwise.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(AggregationExpression... expressions) {
 			return createSetEquals().isEqualTo(expressions);
@@ -113,7 +113,7 @@ public class SetOperators {
 		 * arrays and returns an array that contains the elements that appear in every of those.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public SetIntersection intersects(String... arrayReferences) {
 			return createSetIntersection().intersects(arrayReferences);
@@ -124,7 +124,7 @@ public class SetOperators {
 		 * arrays and returns an array that contains the elements that appear in every of those.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public SetIntersection intersects(AggregationExpression... expressions) {
 			return createSetIntersection().intersects(expressions);
@@ -139,7 +139,7 @@ public class SetOperators {
 		 * arrays and returns an array that contains the elements that appear in any of those.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public SetUnion union(String... arrayReferences) {
 			return createSetUnion().union(arrayReferences);
@@ -150,7 +150,7 @@ public class SetOperators {
 		 * arrays and returns an array that contains the elements that appear in any of those.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public SetUnion union(AggregationExpression... expressions) {
 			return createSetUnion().union(expressions);
@@ -165,7 +165,7 @@ public class SetOperators {
 		 * containing the elements that do not exist in the given {@literal arrayReference}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public SetDifference differenceTo(String arrayReference) {
 			return createSetDifference().differenceTo(arrayReference);
@@ -176,7 +176,7 @@ public class SetOperators {
 		 * containing the elements that do not exist in the given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public SetDifference differenceTo(AggregationExpression expression) {
 			return createSetDifference().differenceTo(expression);
@@ -191,7 +191,7 @@ public class SetOperators {
 		 * {@literal true} if it is a subset of the given {@literal arrayReference}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public SetIsSubset isSubsetOf(String arrayReference) {
 			return createSetIsSubset().isSubsetOf(arrayReference);
@@ -202,7 +202,7 @@ public class SetOperators {
 		 * {@literal true} if it is a subset of the given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public SetIsSubset isSubsetOf(AggregationExpression expression) {
 			return createSetIsSubset().isSubsetOf(expression);
@@ -216,7 +216,7 @@ public class SetOperators {
 		 * Creates new {@link AggregationExpression} that takes array of the previously mentioned field and returns
 		 * {@literal true} if any of the elements are {@literal true} and {@literal false} otherwise.
 		 *
-		 * @return
+		 * @return new instance of {@link AnyElementTrue}.
 		 */
 		public AnyElementTrue anyElementTrue() {
 			return usesFieldRef() ? AnyElementTrue.arrayAsSet(fieldReference) : AnyElementTrue.arrayAsSet(expression);
@@ -226,7 +226,7 @@ public class SetOperators {
 		 * Creates new {@link AggregationExpression} that tkes array of the previously mentioned field and returns
 		 * {@literal true} if no elements is {@literal false}.
 		 *
-		 * @return
+		 * @return new instance of {@link AllElementsTrue}.
 		 */
 		public AllElementsTrue allElementsTrue() {
 			return usesFieldRef() ? AllElementsTrue.arrayAsSet(fieldReference) : AllElementsTrue.arrayAsSet(expression);
@@ -257,7 +257,7 @@ public class SetOperators {
 		 * Create new {@link SetEquals}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public static SetEquals arrayAsSet(String arrayReference) {
 
@@ -269,7 +269,7 @@ public class SetOperators {
 		 * Create new {@link SetEquals}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public static SetEquals arrayAsSet(AggregationExpression expression) {
 
@@ -281,7 +281,7 @@ public class SetOperators {
 		 * Creates new {@link java.util.Set} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(String... arrayReferences) {
 
@@ -293,7 +293,7 @@ public class SetOperators {
 		 * Creates new {@link Sum} with all previously added arguments appending the given one.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(AggregationExpression... expressions) {
 
@@ -305,7 +305,7 @@ public class SetOperators {
 		 * Creates new {@link Sum} with all previously added arguments appending the given one.
 		 *
 		 * @param array must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetEquals}.
 		 */
 		public SetEquals isEqualTo(Object[] array) {
 
@@ -334,7 +334,7 @@ public class SetOperators {
 		 * Creates new {@link SetIntersection}
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public static SetIntersection arrayAsSet(String arrayReference) {
 
@@ -346,7 +346,7 @@ public class SetOperators {
 		 * Creates new {@link SetIntersection}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public static SetIntersection arrayAsSet(AggregationExpression expression) {
 
@@ -358,7 +358,7 @@ public class SetOperators {
 		 * Creates new {@link SetIntersection} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public SetIntersection intersects(String... arrayReferences) {
 
@@ -370,7 +370,7 @@ public class SetOperators {
 		 * Creates new {@link SetIntersection} with all previously added arguments appending the given one.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIntersection}.
 		 */
 		public SetIntersection intersects(AggregationExpression... expressions) {
 
@@ -399,7 +399,7 @@ public class SetOperators {
 		 * Creates new {@link SetUnion}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public static SetUnion arrayAsSet(String arrayReference) {
 
@@ -411,7 +411,7 @@ public class SetOperators {
 		 * Creates new {@link SetUnion}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public static SetUnion arrayAsSet(AggregationExpression expression) {
 
@@ -423,7 +423,7 @@ public class SetOperators {
 		 * Creates new {@link SetUnion} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReferences must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public SetUnion union(String... arrayReferences) {
 
@@ -435,7 +435,7 @@ public class SetOperators {
 		 * Creates new {@link SetUnion} with all previously added arguments appending the given one.
 		 *
 		 * @param expressions must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetUnion}.
 		 */
 		public SetUnion union(AggregationExpression... expressions) {
 
@@ -464,7 +464,7 @@ public class SetOperators {
 		 * Creates new {@link SetDifference}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public static SetDifference arrayAsSet(String arrayReference) {
 
@@ -476,7 +476,7 @@ public class SetOperators {
 		 * Creates new {@link SetDifference}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public static SetDifference arrayAsSet(AggregationExpression expression) {
 
@@ -488,7 +488,7 @@ public class SetOperators {
 		 * Creates new {@link SetDifference} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public SetDifference differenceTo(String arrayReference) {
 
@@ -500,7 +500,7 @@ public class SetOperators {
 		 * Creates new {@link SetDifference} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetDifference}.
 		 */
 		public SetDifference differenceTo(AggregationExpression expression) {
 
@@ -529,7 +529,7 @@ public class SetOperators {
 		 * Creates new {@link SetIsSubset}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public static SetIsSubset arrayAsSet(String arrayReference) {
 
@@ -541,7 +541,7 @@ public class SetOperators {
 		 * Creates new {@link SetIsSubset}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public static SetIsSubset arrayAsSet(AggregationExpression expression) {
 
@@ -553,7 +553,7 @@ public class SetOperators {
 		 * Creates new {@link SetIsSubset} with all previously added arguments appending the given one.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public SetIsSubset isSubsetOf(String arrayReference) {
 
@@ -565,7 +565,7 @@ public class SetOperators {
 		 * Creates new {@link SetIsSubset} with all previously added arguments appending the given one.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SetIsSubset}.
 		 */
 		public SetIsSubset isSubsetOf(AggregationExpression expression) {
 
@@ -594,7 +594,7 @@ public class SetOperators {
 		 * Creates new {@link AnyElementTrue}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AnyElementTrue}.
 		 */
 		public static AnyElementTrue arrayAsSet(String arrayReference) {
 
@@ -606,7 +606,7 @@ public class SetOperators {
 		 * Creates new {@link AnyElementTrue}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AnyElementTrue}.
 		 */
 		public static AnyElementTrue arrayAsSet(AggregationExpression expression) {
 
@@ -639,7 +639,7 @@ public class SetOperators {
 		 * Creates new {@link AllElementsTrue}.
 		 *
 		 * @param arrayReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AllElementsTrue}.
 		 */
 		public static AllElementsTrue arrayAsSet(String arrayReference) {
 
@@ -651,7 +651,7 @@ public class SetOperators {
 		 * Creates new {@link AllElementsTrue}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link AllElementsTrue}.
 		 */
 		public static AllElementsTrue arrayAsSet(AggregationExpression expression) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SkipOperation.java

@@ -28,7 +28,8 @@ import org.springframework.util.Assert;
  * @author Oliver Gierke
  * @author Christoph Strobl
  * @since 1.3
- * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/skip/">MongoDB Aggregation Framework: $skip</a>
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/skip/">MongoDB Aggregation Framework:
+ *      $skip</a>
  */
 public class SkipOperation implements AggregationOperation {
 
@@ -51,6 +52,15 @@ public class SkipOperation implements AggregationOperation {
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
-		return new Document("$skip", skipCount);
+		return new Document(getOperator(), skipCount);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$skip";
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SortByCountOperation.java

@@ -67,14 +67,23 @@ public class SortByCountOperation implements AggregationOperation {
 		this.groupByField = null;
 	}
 
-	/* 
+	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
 	 */
 	@Override
 	public Document toDocument(AggregationOperationContext context) {
 
-		return new Document("$sortByCount", groupByExpression == null ? context.getReference(groupByField).toString()
+		return new Document(getOperator(), groupByExpression == null ? context.getReference(groupByField).toString()
 				: groupByExpression.toDocument(context));
 	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$sortByCount";
+	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/SortOperation.java

@@ -33,7 +33,8 @@ import org.springframework.util.Assert;
  * @author Christoph Strobl
  * @author Mark Paluch
  * @since 1.3
- * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/sort/">MongoDB Aggregation Framework: $sort</a>
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/sort/">MongoDB Aggregation Framework:
+ *      $sort</a>
  */
 public class SortOperation implements AggregationOperation {
 
@@ -74,6 +75,15 @@ public class SortOperation implements AggregationOperation {
 			object.put(reference.getRaw(), order.isAscending() ? 1 : -1);
 		}
 
-		return new Document("$sort", object);
+		return new Document(getOperator(), object);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$sort";
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/StringOperators.java

@@ -35,7 +35,7 @@ public class StringOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link StringOperatorFactory}.
 	 */
 	public static StringOperatorFactory valueOf(String fieldReference) {
 		return new StringOperatorFactory(fieldReference);
@@ -45,7 +45,7 @@ public class StringOperators {
 	 * Take the array referenced by given {@literal fieldReference}.
 	 *
 	 * @param fieldReference must not be {@literal null}.
-	 * @return
+	 * @return new instance of {@link StringOperatorFactory}.
 	 */
 	public static StringOperatorFactory valueOf(AggregationExpression fieldReference) {
 		return new StringOperatorFactory(fieldReference);
@@ -88,7 +88,7 @@ public class StringOperators {
 		 * of the referenced field to it.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public Concat concatValueOf(String fieldReference) {
 
@@ -101,7 +101,7 @@ public class StringOperators {
 		 * of the given {@link AggregationExpression} to it.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public Concat concatValueOf(AggregationExpression expression) {
 
@@ -114,7 +114,7 @@ public class StringOperators {
 		 * {@literal value} to it.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public Concat concat(String value) {
 
@@ -130,8 +130,8 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and returns a substring
 		 * starting at a specified index position.
 		 *
-		 * @param start
-		 * @return
+		 * @param start start index number (including)
+		 * @return new instance of {@link Substr}.
 		 */
 		public Substr substring(int start) {
 			return substring(start, -1);
@@ -141,9 +141,9 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and returns a substring
 		 * starting at a specified index position including the specified number of characters.
 		 *
-		 * @param start
-		 * @param nrOfChars
-		 * @return
+		 * @param start start index number (including)
+		 * @param nrOfChars number of characters.
+		 * @return new instance of {@link Substr}.
 		 */
 		public Substr substring(int start, int nrOfChars) {
 			return createSubstr().substring(start, nrOfChars);
@@ -156,7 +156,7 @@ public class StringOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and lowers it.
 		 *
-		 * @return
+		 * @return new instance of {@link ToLower}.
 		 */
 		public ToLower toLower() {
 			return usesFieldRef() ? ToLower.lowerValueOf(fieldReference) : ToLower.lowerValueOf(expression);
@@ -165,7 +165,7 @@ public class StringOperators {
 		/**
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and uppers it.
 		 *
-		 * @return
+		 * @return new instance of {@link ToUpper}.
 		 */
 		public ToUpper toUpper() {
 			return usesFieldRef() ? ToUpper.upperValueOf(fieldReference) : ToUpper.upperValueOf(expression);
@@ -176,7 +176,7 @@ public class StringOperators {
 		 * case-insensitive comparison to the given {@literal value}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public StrCaseCmp strCaseCmp(String value) {
 
@@ -189,7 +189,7 @@ public class StringOperators {
 		 * case-insensitive comparison to the referenced {@literal fieldReference}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public StrCaseCmp strCaseCmpValueOf(String fieldReference) {
 
@@ -202,7 +202,7 @@ public class StringOperators {
 		 * case-insensitive comparison to the result of the given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public StrCaseCmp strCaseCmpValueOf(AggregationExpression expression) {
 
@@ -220,7 +220,7 @@ public class StringOperators {
 		 * occurrence.
 		 *
 		 * @param substring must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfBytes}.
 		 */
 		public IndexOfBytes indexOf(String substring) {
 
@@ -234,7 +234,7 @@ public class StringOperators {
 		 * index (zero-based) of the first occurrence.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfBytes}.
 		 */
 		public IndexOfBytes indexOf(Field fieldReference) {
 
@@ -248,7 +248,7 @@ public class StringOperators {
 		 * byte index (zero-based) of the first occurrence.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfBytes}.
 		 */
 		public IndexOfBytes indexOf(AggregationExpression expression) {
 
@@ -266,7 +266,7 @@ public class StringOperators {
 		 * first occurrence.
 		 *
 		 * @param substring must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public IndexOfCP indexOfCP(String substring) {
 
@@ -280,7 +280,7 @@ public class StringOperators {
 		 * point index (zero-based) of the first occurrence.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public IndexOfCP indexOfCP(Field fieldReference) {
 
@@ -294,7 +294,7 @@ public class StringOperators {
 		 * code point index (zero-based) of the first occurrence.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public IndexOfCP indexOfCP(AggregationExpression expression) {
 
@@ -311,7 +311,7 @@ public class StringOperators {
 		 * substrings based on the given delimiter.
 		 *
 		 * @param delimiter must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(String delimiter) {
 			return createSplit().split(delimiter);
@@ -322,7 +322,7 @@ public class StringOperators {
 		 * substrings based on the delimiter resulting from the referenced field..
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(Field fieldReference) {
 			return createSplit().split(fieldReference);
@@ -333,7 +333,7 @@ public class StringOperators {
 		 * substrings based on a delimiter resulting from the given {@link AggregationExpression}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(AggregationExpression expression) {
 			return createSplit().split(expression);
@@ -347,7 +347,7 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that returns the number of UTF-8 bytes in the associated string
 		 * representation.
 		 *
-		 * @return
+		 * @return new instance of {@link StrLenBytes}.
 		 */
 		public StrLenBytes length() {
 			return usesFieldRef() ? StrLenBytes.stringLengthOf(fieldReference) : StrLenBytes.stringLengthOf(expression);
@@ -357,7 +357,7 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that returns the number of UTF-8 code points in the associated string
 		 * representation.
 		 *
-		 * @return
+		 * @return new instance of {@link StrLenCP}.
 		 */
 		public StrLenCP lengthCP() {
 			return usesFieldRef() ? StrLenCP.stringLengthOfCP(fieldReference) : StrLenCP.stringLengthOfCP(expression);
@@ -368,7 +368,7 @@ public class StringOperators {
 		 * starting at a specified code point index position.
 		 *
 		 * @param codePointStart
-		 * @return
+		 * @return new instance of {@link SubstrCP}.
 		 */
 		public SubstrCP substringCP(int codePointStart) {
 			return substringCP(codePointStart, -1);
@@ -378,9 +378,9 @@ public class StringOperators {
 		 * Creates new {@link AggregationExpression} that takes the associated string representation and returns a substring
 		 * starting at a specified code point index position including the specified number of code points.
 		 *
-		 * @param codePointStart
+		 * @param codePointStart start point (including).
 		 * @param nrOfCodePoints
-		 * @return
+		 * @return new instance of {@link SubstrCP}.
 		 */
 		public SubstrCP substringCP(int codePointStart, int nrOfCodePoints) {
 			return createSubstrCP().substringCP(codePointStart, nrOfCodePoints);
@@ -541,7 +541,7 @@ public class StringOperators {
 		 * Creates new {@link Concat}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public static Concat valueOf(String fieldReference) {
 
@@ -553,7 +553,7 @@ public class StringOperators {
 		 * Creates new {@link Concat}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public static Concat valueOf(AggregationExpression expression) {
 
@@ -565,7 +565,7 @@ public class StringOperators {
 		 * Creates new {@link Concat}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Concat}.
 		 */
 		public static Concat stringValue(String value) {
 
@@ -573,18 +573,36 @@ public class StringOperators {
 			return new Concat(Collections.singletonList(value));
 		}
 
+		/**
+		 * Concat the value of the given field.
+		 *
+		 * @param fieldReference must not be {@literal null}.
+		 * @return new instance of {@link Concat}.
+		 */
 		public Concat concatValueOf(String fieldReference) {
 
 			Assert.notNull(fieldReference, "FieldReference must not be null!");
 			return new Concat(append(Fields.field(fieldReference)));
 		}
 
+		/**
+		 * Concat the value resulting from the given {@link AggregationExpression}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link Concat}.
+		 */
 		public Concat concatValueOf(AggregationExpression expression) {
 
 			Assert.notNull(expression, "Expression must not be null!");
 			return new Concat(append(expression));
 		}
 
+		/**
+		 * Concat the given value.
+		 *
+		 * @param value must not be {@literal null}.
+		 * @return new instance of {@link Concat}.
+		 */
 		public Concat concat(String value) {
 			return new Concat(append(value));
 		}
@@ -610,7 +628,7 @@ public class StringOperators {
 		 * Creates new {@link Substr}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Substr}.
 		 */
 		public static Substr valueOf(String fieldReference) {
 
@@ -622,7 +640,7 @@ public class StringOperators {
 		 * Creates new {@link Substr}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Substr}.
 		 */
 		public static Substr valueOf(AggregationExpression expression) {
 
@@ -630,10 +648,19 @@ public class StringOperators {
 			return new Substr(Collections.singletonList(expression));
 		}
 
+		/**
+		 * @param start start index (including)
+		 * @return new instance of {@link Substr}.
+		 */
 		public Substr substring(int start) {
 			return substring(start, -1);
 		}
 
+		/**
+		 * @param start start index (including)
+		 * @param nrOfChars
+		 * @return new instance of {@link Substr}.
+		 */
 		public Substr substring(int start, int nrOfChars) {
 			return new Substr(append(Arrays.asList(start, nrOfChars)));
 		}
@@ -777,7 +804,7 @@ public class StringOperators {
 		 * Creates new {@link StrCaseCmp}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public static StrCaseCmp valueOf(AggregationExpression expression) {
 
@@ -789,7 +816,7 @@ public class StringOperators {
 		 * Creates new {@link StrCaseCmp}.
 		 *
 		 * @param value must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrCaseCmp}.
 		 */
 		public static StrCaseCmp stringValue(String value) {
 
@@ -834,7 +861,7 @@ public class StringOperators {
 		 * Start creating a new {@link IndexOfBytes}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SubstringBuilder}.
 		 */
 		public static SubstringBuilder valueOf(String fieldReference) {
 
@@ -846,7 +873,7 @@ public class StringOperators {
 		 * Start creating a new {@link IndexOfBytes}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SubstringBuilder}.
 		 */
 		public static SubstringBuilder valueOf(AggregationExpression expression) {
 
@@ -858,7 +885,7 @@ public class StringOperators {
 		 * Optionally define the substring search start and end position.
 		 *
 		 * @param range must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfBytes}.
 		 */
 		public IndexOfBytes within(Range<Long> range) {
 			return new IndexOfBytes(append(AggregationUtils.toRangeValues(range)));
@@ -876,7 +903,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfBytes} given {@literal substring}.
 			 *
 			 * @param substring must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfBytes}.
 			 */
 			public IndexOfBytes indexOf(String substring) {
 				return new IndexOfBytes(Arrays.asList(stringExpression, substring));
@@ -886,7 +913,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfBytes} given {@link AggregationExpression} that resolves to the substring.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfBytes}.
 			 */
 			public IndexOfBytes indexOf(AggregationExpression expression) {
 				return new IndexOfBytes(Arrays.asList(stringExpression, expression));
@@ -896,7 +923,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfBytes} given {@link Field} that resolves to the substring.
 			 *
 			 * @param fieldReference must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfBytes}.
 			 */
 			public IndexOfBytes indexOf(Field fieldReference) {
 				return new IndexOfBytes(Arrays.asList(stringExpression, fieldReference));
@@ -924,7 +951,7 @@ public class StringOperators {
 		 * Start creating a new {@link IndexOfCP}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public static SubstringBuilder valueOf(String fieldReference) {
 
@@ -936,7 +963,7 @@ public class StringOperators {
 		 * Start creating a new {@link IndexOfCP}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public static SubstringBuilder valueOf(AggregationExpression expression) {
 
@@ -948,7 +975,7 @@ public class StringOperators {
 		 * Optionally define the substring search start and end position.
 		 *
 		 * @param range must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link IndexOfCP}.
 		 */
 		public IndexOfCP within(Range<Long> range) {
 			return new IndexOfCP(append(AggregationUtils.toRangeValues(range)));
@@ -966,7 +993,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfCP} given {@literal substring}.
 			 *
 			 * @param substring must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfCP}.
 			 */
 			public IndexOfCP indexOf(String substring) {
 				return new IndexOfCP(Arrays.asList(stringExpression, substring));
@@ -976,7 +1003,7 @@ public class StringOperators {
 			 * Creates a new {@link IndexOfCP} given {@link AggregationExpression} that resolves to the substring.
 			 *
 			 * @param expression must not be {@literal null}.
-			 * @return
+			 * @return new instance of {@link IndexOfCP}.
 			 */
 			public IndexOfCP indexOf(AggregationExpression expression) {
 				return new IndexOfCP(Arrays.asList(stringExpression, expression));
@@ -1014,7 +1041,7 @@ public class StringOperators {
 		 * Start creating a new {@link Split}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public static Split valueOf(String fieldReference) {
 
@@ -1026,7 +1053,7 @@ public class StringOperators {
 		 * Start creating a new {@link Split}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public static Split valueOf(AggregationExpression expression) {
 
@@ -1038,7 +1065,7 @@ public class StringOperators {
 		 * Use given {@link String} as delimiter.
 		 *
 		 * @param delimiter must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(String delimiter) {
 
@@ -1050,7 +1077,7 @@ public class StringOperators {
 		 * Use value of referenced field as delimiter.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(Field fieldReference) {
 
@@ -1062,7 +1089,7 @@ public class StringOperators {
 		 * Use value resulting from {@link AggregationExpression} as delimiter.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link Split}.
 		 */
 		public Split split(AggregationExpression expression) {
 
@@ -1091,7 +1118,7 @@ public class StringOperators {
 		 * Creates new {@link StrLenBytes}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrLenBytes}.
 		 */
 		public static StrLenBytes stringLengthOf(String fieldReference) {
 			return new StrLenBytes(Fields.field(fieldReference));
@@ -1101,7 +1128,7 @@ public class StringOperators {
 		 * Creates new {@link StrLenBytes}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrLenBytes}.
 		 */
 		public static StrLenBytes stringLengthOf(AggregationExpression expression) {
 
@@ -1130,7 +1157,7 @@ public class StringOperators {
 		 * Creates new {@link StrLenCP}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrLenCP}.
 		 */
 		public static StrLenCP stringLengthOfCP(String fieldReference) {
 			return new StrLenCP(Fields.field(fieldReference));
@@ -1140,7 +1167,7 @@ public class StringOperators {
 		 * Creates new {@link StrLenCP}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link StrLenCP}.
 		 */
 		public static StrLenCP stringLengthOfCP(AggregationExpression expression) {
 
@@ -1169,7 +1196,7 @@ public class StringOperators {
 		 * Creates new {@link SubstrCP}.
 		 *
 		 * @param fieldReference must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SubstrCP}.
 		 */
 		public static SubstrCP valueOf(String fieldReference) {
 
@@ -1181,7 +1208,7 @@ public class StringOperators {
 		 * Creates new {@link SubstrCP}.
 		 *
 		 * @param expression must not be {@literal null}.
-		 * @return
+		 * @return new instance of {@link SubstrCP}.
 		 */
 		public static SubstrCP valueOf(AggregationExpression expression) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/TypedAggregation.java

@@ -81,6 +81,6 @@ public class TypedAggregation<I> extends Aggregation {
 	public TypedAggregation<I> withOptions(AggregationOptions options) {
 
 		Assert.notNull(options, "AggregationOptions must not be null.");
-		return new TypedAggregation<I>(inputType, operations, options);
+		return new TypedAggregation<I>(inputType, pipeline.getOperations(), options);
 	}
 }

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/UnsetOperation.java

@@ -40,7 +40,7 @@ public class UnsetOperation implements InheritsFieldsAggregationOperation {
 
 	/**
 	 * Create new instance of {@link UnsetOperation}.
-	 * 
+	 *
 	 * @param fields must not be {@literal null}.
 	 */
 	public UnsetOperation(Collection<Object> fields) {
@@ -117,13 +117,22 @@ public class UnsetOperation implements InheritsFieldsAggregationOperation {
 	public Document toDocument(AggregationOperationContext context) {
 
 		if (fields.size() == 1) {
-			return new Document("$unset", computeFieldName(fields.iterator().next(), context));
+			return new Document(getOperator(), computeFieldName(fields.iterator().next(), context));
 		}
 
-		return new Document("$unset",
+		return new Document(getOperator(),
 				fields.stream().map(it -> computeFieldName(it, context)).collect(Collectors.toList()));
 	}
 
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$unset";
+	}
+
 	private Object computeFieldName(Object field, AggregationOperationContext context) {
 
 		if (field instanceof Field) {

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/UnwindOperation.java

@@ -31,7 +31,8 @@ import org.springframework.util.Assert;
  * @author Mark Paluch
  * @author Christoph Strobl
  * @since 1.3
- * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/">MongoDB Aggregation Framework: $unwind</a>
+ * @see <a href="https://docs.mongodb.com/manual/reference/operator/aggregation/unwind/">MongoDB Aggregation Framework:
+ *      $unwind</a>
  */
 public class UnwindOperation
 		implements AggregationOperation, FieldsExposingAggregationOperation.InheritsFieldsAggregationOperation {
@@ -94,7 +95,7 @@ public class UnwindOperation
 		String path = context.getReference(field).toString();
 
 		if (!preserveNullAndEmptyArrays && arrayIndex == null) {
-			return new Document("$unwind", path);
+			return new Document(getOperator(), path);
 		}
 
 		Document unwindArgs = new Document();
@@ -104,7 +105,16 @@ public class UnwindOperation
 		}
 		unwindArgs.put("preserveNullAndEmptyArrays", preserveNullAndEmptyArrays);
 
-		return new Document("$unwind", unwindArgs);
+		return new Document(getOperator(), unwindArgs);
+	}
+
+	/* 
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$unwind";
 	}
 
 	/*

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/AbstractMongoConverter.java

@@ -72,7 +72,7 @@ public abstract class AbstractMongoConverter implements MongoConverter, Initiali
 	/**
 	 * Registers {@link EntityInstantiators} to customize entity instantiation.
 	 *
-	 * @param instantiators
+	 * @param instantiators can be {@literal null}. Uses default {@link EntityInstantiators} if so.
 	 */
 	public void setInstantiators(@Nullable EntityInstantiators instantiators) {
 		this.instantiators = instantiators == null ? new EntityInstantiators() : instantiators;

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/CustomConversions.java

@@ -46,7 +46,7 @@ public class CustomConversions extends MongoCustomConversions {
 	/**
 	 * Creates a new {@link CustomConversions} instance registering the given converters.
 	 *
-	 * @param converters
+	 * @param converters must not be {@literal null}.
 	 */
 	public CustomConversions(List<?> converters) {
 		super(converters);

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/convert/DbRefResolver.java

@@ -45,7 +45,7 @@ public interface DbRefResolver {
 	 * @param property will never be {@literal null}.
 	 * @param dbref the {@link DBRef} to resolve.
 	 * @param callback will never be {@literal null}.
-	 * @return
+	 * @return can be {@literal null}.
 	 */
 	@Nullable
 	Object resolveDbRef(MongoPersistentProperty property, @Nullable DBRef dbref, DbRefResolverCallback callback,
@@ -58,7 +58,7 @@ public interface DbRefResolver {
 	 * @param annotation will never be {@literal null}.
 	 * @param entity will never be {@literal null}.
 	 * @param id will never be {@literal null}.
-	 * @return
+	 * @return new instance of {@link DBRef}.
 	 */
 	default DBRef createDbRef(@Nullable org.springframework.data.mongodb.core.mapping.DBRef annotation,
 			MongoPersistentEntity<?> entity, Object id) {
@@ -74,7 +74,7 @@ public interface DbRefResolver {
 	 * Actually loads the {@link DBRef} from the datasource.
 	 *
 	 * @param dbRef must not be {@literal null}.
-	 * @return
+	 * @return can be {@literal null}.
 	 * @since 1.7
 	 */
 	@Nullable