switch to commons branch

Reformat code. Merge if-statements.

Original pull request: #889.

CODE_OF_CONDUCT.adoc

@@ -1,27 +0,0 @@
-= Contributor Code of Conduct
-
-As contributors and maintainers of this project, and in the interest of fostering an open and welcoming community, we pledge to respect all people who contribute through reporting issues, posting feature requests, updating documentation, submitting pull requests or patches, and other activities.
-
-We are committed to making participation in this project a harassment-free experience for everyone, regardless of level of experience, gender, gender identity and expression, sexual orientation, disability, personal appearance, body size, race, ethnicity, age, religion, or nationality.
-
-Examples of unacceptable behavior by participants include:
-
-* The use of sexualized language or imagery
-* Personal attacks
-* Trolling or insulting/derogatory comments
-* Public or private harassment
-* Publishing other's private information, such as physical or electronic addresses,
-  without explicit permission
-* Other unethical or unprofessional conduct
-
-Project maintainers have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, or to ban temporarily or permanently any contributor for other behaviors that they deem inappropriate, threatening, offensive, or harmful.
-
-By adopting this Code of Conduct, project maintainers commit themselves to fairly and consistently applying these principles to every aspect of managing this project. Project maintainers who do not follow or enforce the Code of Conduct may be permanently removed from the project team.
-
-This Code of Conduct applies both within project spaces and in public spaces when an individual is representing the project or its community.
-
-Instances of abusive, harassing, or otherwise unacceptable behavior may be reported by contacting a project maintainer at spring-code-of-conduct@pivotal.io.
-All complaints will be reviewed and investigated and will result in a response that is deemed necessary and appropriate to the circumstances.
-Maintainers are obligated to maintain confidentiality with regard to the reporter of an incident.
-
-This Code of Conduct is adapted from the https://contributor-covenant.org[Contributor Covenant], version 1.3.0, available at https://contributor-covenant.org/version/1/3/0/[contributor-covenant.org/version/1/3/0/].

Jenkinsfile

@@ -93,7 +93,7 @@ pipeline {
 
 		stage("Test other configurations") {
 			when {
-				anyOf {
+				allOf {
 					branch 'master'
 					not { triggeredBy 'UpstreamCause' }
 				}

README.adoc

@@ -10,7 +10,7 @@ Key functional areas of Spring Data MongoDB are a POJO centric model for interac
 
 == Code of Conduct
 
-This project is governed by the link:CODE_OF_CONDUCT.adoc[Spring Code of Conduct]. By participating, you are expected to uphold this code of conduct. Please report unacceptable behavior to spring-code-of-conduct@pivotal.io.
+This project is governed by the https://github.com/spring-projects/.github/blob/e3cc2ff230d8f1dca06535aa6b5a4a23815861d4/CODE_OF_CONDUCT.md[Spring Code of Conduct]. By participating, you are expected to uphold this code of conduct. Please report unacceptable behavior to spring-code-of-conduct@pivotal.io.
 
 == Getting Started
 

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>3.0.0.RELEASE</version>
+	<version>3.1.0-STATIC-METADATA-SNAPSHOT</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.RELEASE</version>
+		<version>2.4.0-SNAPSHOT</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.RELEASE</springdata.commons>
-		<mongo>4.0.3</mongo>
+		<springdata.commons>2.4.0-BUILD-TIME-DOMAIN-TYPE-METADATA-SNAPSHOT</springdata.commons>
+		<mongo>4.1.0</mongo>
 		<mongo.reactivestreams>${mongo}</mongo.reactivestreams>
 		<jmh.version>1.19</jmh.version>
 	</properties>
@@ -134,8 +134,8 @@
 
 	<repositories>
 		<repository>
-			<id>spring-libs-release</id>
-			<url>https://repo.spring.io/libs-release</url>
+			<id>spring-libs-snapshot</id>
+			<url>https://repo.spring.io/libs-snapshot</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.RELEASE</version>
+		<version>3.1.0-STATIC-METADATA-SNAPSHOT</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.RELEASE</version>
+		<version>3.1.0-STATIC-METADATA-SNAPSHOT</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.RELEASE</version>
+		<version>3.1.0-STATIC-METADATA-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 
@@ -136,6 +136,13 @@
 			<optional>true</optional>
 		</dependency>
 
+		<dependency>
+			<groupId>io.reactivex.rxjava3</groupId>
+			<artifactId>rxjava</artifactId>
+			<version>${rxjava3}</version>
+			<optional>true</optional>
+		</dependency>
+
 		<!-- CDI -->
 		<!-- Dependency order required to build against CDI 1.0 and test with CDI 2.0 -->
 		<dependency>
@@ -192,7 +199,14 @@
 		<dependency>
 			<groupId>org.hibernate</groupId>
 			<artifactId>hibernate-validator</artifactId>
-			<version>5.2.4.Final</version>
+			<version>5.4.3.Final</version>
+			<scope>test</scope>
+		</dependency>
+
+		<dependency>
+			<groupId>org.glassfish</groupId>
+			<artifactId>javax.el</artifactId>
+			<version>3.0.1-b11</version>
 			<scope>test</scope>
 		</dependency>
 

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/config/EnableMongoAuditing.java

@@ -61,8 +61,8 @@ public @interface EnableMongoAuditing {
 	boolean modifyOnCreate() default true;
 
 	/**
-	 * Configures a {@link DateTimeProvider} bean name that allows customizing the {@link org.joda.time.DateTime} to be
-	 * used for setting creation and modification dates.
+	 * Configures a {@link DateTimeProvider} bean name that allows customizing the timestamp to be used for setting
+	 * creation and modification dates.
 	 *
 	 * @return empty {@link String} by default.
 	 */

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

@@ -0,0 +1,70 @@
+/*
+ * 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.config;
+
+import java.lang.annotation.Documented;
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Inherited;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+import org.springframework.context.annotation.Import;
+import org.springframework.data.auditing.DateTimeProvider;
+import org.springframework.data.domain.ReactiveAuditorAware;
+
+/**
+ * Annotation to enable auditing in MongoDB using reactive infrastructure via annotation configuration.
+ *
+ * @author Mark Paluch
+ * @since 3.1
+ */
+@Inherited
+@Documented
+@Target(ElementType.TYPE)
+@Retention(RetentionPolicy.RUNTIME)
+@Import(ReactiveMongoAuditingRegistrar.class)
+public @interface EnableReactiveMongoAuditing {
+
+	/**
+	 * Configures the {@link ReactiveAuditorAware} bean to be used to lookup the current principal.
+	 *
+	 * @return empty {@link String} by default.
+	 */
+	String auditorAwareRef() default "";
+
+	/**
+	 * Configures whether the creation and modification dates are set. Defaults to {@literal true}.
+	 *
+	 * @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 {@literal true} by default.
+	 */
+	boolean modifyOnCreate() default true;
+
+	/**
+	 * Configures a {@link DateTimeProvider} bean name that allows customizing the timestamp to be used for setting
+	 * creation and modification dates.
+	 *
+	 * @return empty {@link String} by default.
+	 */
+	String dateTimeProviderRef() default "";
+}

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

@@ -17,7 +17,6 @@ package org.springframework.data.mongodb.config;
 
 import java.lang.annotation.Annotation;
 
-import org.springframework.beans.factory.FactoryBean;
 import org.springframework.beans.factory.config.BeanDefinition;
 import org.springframework.beans.factory.support.AbstractBeanDefinition;
 import org.springframework.beans.factory.support.BeanDefinitionBuilder;
@@ -28,14 +27,8 @@ import org.springframework.data.auditing.IsNewAwareAuditingHandler;
 import org.springframework.data.auditing.config.AuditingBeanDefinitionRegistrarSupport;
 import org.springframework.data.auditing.config.AuditingConfiguration;
 import org.springframework.data.config.ParsingUtils;
-import org.springframework.data.mapping.context.MappingContext;
-import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
-import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
-import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.data.mongodb.core.mapping.event.AuditingEntityCallback;
-import org.springframework.data.mongodb.core.mapping.event.ReactiveAuditingEntityCallback;
 import org.springframework.util.Assert;
-import org.springframework.util.ClassUtils;
 
 /**
  * {@link ImportBeanDefinitionRegistrar} to enable {@link EnableMongoAuditing} annotation.
@@ -46,9 +39,6 @@ import org.springframework.util.ClassUtils;
  */
 class MongoAuditingRegistrar extends AuditingBeanDefinitionRegistrarSupport {
 
-	private static boolean PROJECT_REACTOR_AVAILABLE = ClassUtils.isPresent("reactor.core.publisher.Mono",
-			MongoAuditingRegistrar.class.getClassLoader());
-
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.auditing.config.AuditingBeanDefinitionRegistrarSupport#getAnnotation()
@@ -91,7 +81,7 @@ class MongoAuditingRegistrar extends AuditingBeanDefinitionRegistrarSupport {
 
 		BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(IsNewAwareAuditingHandler.class);
 
-		BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(MongoMappingContextLookup.class);
+		BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(PersistentEntitiesFactoryBean.class);
 		definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_CONSTRUCTOR);
 
 		builder.addConstructorArgValue(definition.getBeanDefinition());
@@ -116,68 +106,6 @@ class MongoAuditingRegistrar extends AuditingBeanDefinitionRegistrarSupport {
 
 		registerInfrastructureBeanWithId(listenerBeanDefinitionBuilder.getBeanDefinition(),
 				AuditingEntityCallback.class.getName(), registry);
-
-		if (PROJECT_REACTOR_AVAILABLE) {
-			registerReactiveAuditingEntityCallback(registry, auditingHandlerDefinition.getSource());
-		}
 	}
 
-	private void registerReactiveAuditingEntityCallback(BeanDefinitionRegistry registry, Object source) {
-
-		BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(ReactiveAuditingEntityCallback.class);
-
-		builder.addConstructorArgValue(ParsingUtils.getObjectFactoryBeanDefinition(getAuditingHandlerBeanName(), registry));
-		builder.getRawBeanDefinition().setSource(source);
-
-		registerInfrastructureBeanWithId(builder.getBeanDefinition(), ReactiveAuditingEntityCallback.class.getName(),
-				registry);
-	}
-
-	/**
-	 * Simple helper to be able to wire the {@link MappingContext} from a {@link MappingMongoConverter} bean available in
-	 * the application context.
-	 *
-	 * @author Oliver Gierke
-	 */
-	static class MongoMappingContextLookup
-			implements FactoryBean<MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty>> {
-
-		private final MappingMongoConverter converter;
-
-		/**
-		 * Creates a new {@link MongoMappingContextLookup} for the given {@link MappingMongoConverter}.
-		 *
-		 * @param converter must not be {@literal null}.
-		 */
-		public MongoMappingContextLookup(MappingMongoConverter converter) {
-			this.converter = converter;
-		}
-
-		/*
-		 * (non-Javadoc)
-		 * @see org.springframework.beans.factory.FactoryBean#getObject()
-		 */
-		@Override
-		public MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> getObject() throws Exception {
-			return converter.getMappingContext();
-		}
-
-		/*
-		 * (non-Javadoc)
-		 * @see org.springframework.beans.factory.FactoryBean#getObjectType()
-		 */
-		@Override
-		public Class<?> getObjectType() {
-			return MappingContext.class;
-		}
-
-		/*
-		 * (non-Javadoc)
-		 * @see org.springframework.beans.factory.FactoryBean#isSingleton()
-		 */
-		@Override
-		public boolean isSingleton() {
-			return true;
-		}
-	}
 }

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

@@ -0,0 +1,61 @@
+/*
+ * 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.config;
+
+import org.springframework.beans.factory.FactoryBean;
+import org.springframework.data.mapping.context.PersistentEntities;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
+
+/**
+ * Simple helper to be able to wire the {@link PersistentEntities} from a {@link MappingMongoConverter} bean available
+ * in the application context.
+ *
+ * @author Oliver Gierke
+ * @author Mark Paluch
+ * @author Christoph Strobl
+ * @since 3.1
+ */
+class PersistentEntitiesFactoryBean implements FactoryBean<PersistentEntities> {
+
+	private final MappingMongoConverter converter;
+
+	/**
+	 * Creates a new {@link PersistentEntitiesFactoryBean} for the given {@link MappingMongoConverter}.
+	 *
+	 * @param converter must not be {@literal null}.
+	 */
+	public PersistentEntitiesFactoryBean(MappingMongoConverter converter) {
+		this.converter = converter;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.beans.factory.FactoryBean#getObject()
+	 */
+	@Override
+	public PersistentEntities getObject() {
+		return PersistentEntities.of(converter.getMappingContext());
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.beans.factory.FactoryBean#getObjectType()
+	 */
+	@Override
+	public Class<?> getObjectType() {
+		return PersistentEntities.class;
+	}
+}

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

@@ -0,0 +1,97 @@
+/*
+ * 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.config;
+
+import java.lang.annotation.Annotation;
+
+import org.springframework.beans.factory.config.BeanDefinition;
+import org.springframework.beans.factory.support.AbstractBeanDefinition;
+import org.springframework.beans.factory.support.BeanDefinitionBuilder;
+import org.springframework.beans.factory.support.BeanDefinitionRegistry;
+import org.springframework.context.annotation.ImportBeanDefinitionRegistrar;
+import org.springframework.core.type.AnnotationMetadata;
+import org.springframework.data.auditing.ReactiveIsNewAwareAuditingHandler;
+import org.springframework.data.auditing.config.AuditingBeanDefinitionRegistrarSupport;
+import org.springframework.data.auditing.config.AuditingConfiguration;
+import org.springframework.data.config.ParsingUtils;
+import org.springframework.data.mongodb.core.mapping.event.ReactiveAuditingEntityCallback;
+import org.springframework.util.Assert;
+
+/**
+ * {@link ImportBeanDefinitionRegistrar} to enable {@link EnableReactiveMongoAuditing} annotation.
+ *
+ * @author Mark Paluch
+ * @since 3.1
+ */
+class ReactiveMongoAuditingRegistrar extends AuditingBeanDefinitionRegistrarSupport {
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.auditing.config.AuditingBeanDefinitionRegistrarSupport#getAnnotation()
+	 */
+	@Override
+	protected Class<? extends Annotation> getAnnotation() {
+		return EnableReactiveMongoAuditing.class;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.auditing.config.AuditingBeanDefinitionRegistrarSupport#getAuditingHandlerBeanName()
+	 */
+	@Override
+	protected String getAuditingHandlerBeanName() {
+		return "reactiveMongoAuditingHandler";
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.auditing.config.AuditingBeanDefinitionRegistrarSupport#getAuditHandlerBeanDefinitionBuilder(org.springframework.data.auditing.config.AuditingConfiguration)
+	 */
+	@Override
+	protected BeanDefinitionBuilder getAuditHandlerBeanDefinitionBuilder(AuditingConfiguration configuration) {
+
+		Assert.notNull(configuration, "AuditingConfiguration must not be null!");
+
+		BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(ReactiveIsNewAwareAuditingHandler.class);
+
+		BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(PersistentEntitiesFactoryBean.class);
+		definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_CONSTRUCTOR);
+
+		builder.addConstructorArgValue(definition.getBeanDefinition());
+		return configureDefaultAuditHandlerAttributes(configuration, builder);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.auditing.config.AuditingBeanDefinitionRegistrarSupport#registerAuditListener(org.springframework.beans.factory.config.BeanDefinition, org.springframework.beans.factory.support.BeanDefinitionRegistry)
+	 */
+	@Override
+	protected void registerAuditListenerBeanDefinition(BeanDefinition auditingHandlerDefinition,
+			BeanDefinitionRegistry registry) {
+
+		Assert.notNull(auditingHandlerDefinition, "BeanDefinition must not be null!");
+		Assert.notNull(registry, "BeanDefinitionRegistry must not be null!");
+
+		BeanDefinitionBuilder builder = BeanDefinitionBuilder.rootBeanDefinition(ReactiveAuditingEntityCallback.class);
+
+		builder.addConstructorArgValue(ParsingUtils.getObjectFactoryBeanDefinition(getAuditingHandlerBeanName(), registry));
+		builder.getRawBeanDefinition().setSource(auditingHandlerDefinition.getSource());
+
+		registerInfrastructureBeanWithId(builder.getBeanDefinition(), ReactiveAuditingEntityCallback.class.getName(),
+				registry);
+	}
+
+}

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AllArgsConstructor;
-
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
@@ -30,6 +28,7 @@ import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions;
 import org.springframework.data.mongodb.core.aggregation.CountOperation;
+import org.springframework.data.mongodb.core.aggregation.RelaxedTypeBasedAggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.TypeBasedAggregationOperationContext;
 import org.springframework.data.mongodb.core.aggregation.TypedAggregation;
 import org.springframework.data.mongodb.core.convert.QueryMapper;
@@ -49,12 +48,18 @@ import org.springframework.util.ObjectUtils;
  * @author Mark Paluch
  * @since 2.1
  */
-@AllArgsConstructor
 class AggregationUtil {
 
 	QueryMapper queryMapper;
 	MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext;
 
+	AggregationUtil(QueryMapper queryMapper,
+			MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext) {
+
+		this.queryMapper = queryMapper;
+		this.mappingContext = mappingContext;
+	}
+
 	/**
 	 * Prepare the {@link AggregationOperationContext} for a given aggregation by either returning the context itself it
 	 * is not {@literal null}, create a {@link TypeBasedAggregationOperationContext} if the aggregation contains type
@@ -71,12 +76,17 @@ class AggregationUtil {
 			return context;
 		}
 
-		if (aggregation instanceof TypedAggregation) {
-			return new TypeBasedAggregationOperationContext(((TypedAggregation) aggregation).getInputType(), mappingContext,
-					queryMapper);
+		if (!(aggregation instanceof TypedAggregation)) {
+			return Aggregation.DEFAULT_CONTEXT;
 		}
 
-		return Aggregation.DEFAULT_CONTEXT;
+		Class<?> inputType = ((TypedAggregation) aggregation).getInputType();
+
+		if (aggregation.getPipeline().containsUnionWith()) {
+			return new RelaxedTypeBasedAggregationOperationContext(inputType, mappingContext, queryMapper);
+		}
+
+		return new TypeBasedAggregationOperationContext(inputType, mappingContext, queryMapper);
 	}
 
 	/**

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.EqualsAndHashCode;
-
 import java.time.Instant;
 import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
 
@@ -27,6 +25,7 @@ import org.springframework.data.mongodb.core.convert.MongoConverter;
 import org.springframework.data.mongodb.core.messaging.Message;
 import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.ObjectUtils;
 
 import com.mongodb.client.model.changestream.ChangeStreamDocument;
 import com.mongodb.client.model.changestream.OperationType;
@@ -39,7 +38,6 @@ import com.mongodb.client.model.changestream.OperationType;
  * @author Mark Paluch
  * @since 2.1
  */
-@EqualsAndHashCode
 public class ChangeStreamEvent<T> {
 
 	@SuppressWarnings("rawtypes") //
@@ -187,8 +185,8 @@ public class ChangeStreamEvent<T> {
 			return CONVERTED_UPDATER.compareAndSet(this, null, result) ? result : CONVERTED_UPDATER.get(this);
 		}
 
-		throw new IllegalArgumentException(String.format("No converter found capable of converting %s to %s",
-				fullDocument.getClass(), targetType));
+		throw new IllegalArgumentException(
+				String.format("No converter found capable of converting %s to %s", fullDocument.getClass(), targetType));
 	}
 
 	/*
@@ -199,4 +197,27 @@ public class ChangeStreamEvent<T> {
 	public String toString() {
 		return "ChangeStreamEvent {" + "raw=" + raw + ", targetType=" + targetType + '}';
 	}
+
+	@Override
+	public boolean equals(Object o) {
+
+		if (this == o)
+			return true;
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		ChangeStreamEvent<?> that = (ChangeStreamEvent<?>) o;
+
+		if (!ObjectUtils.nullSafeEquals(this.raw, that.raw)) {
+			return false;
+		}
+		return ObjectUtils.nullSafeEquals(this.targetType, that.targetType);
+	}
+
+	@Override
+	public int hashCode() {
+		int result = raw != null ? raw.hashCode() : 0;
+		result = 31 * result + ObjectUtils.nullSafeHashCode(targetType);
+		return result;
+	}
 }

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.EqualsAndHashCode;
-
 import java.time.Instant;
 import java.util.Arrays;
 import java.util.Optional;
@@ -25,7 +23,6 @@ import org.bson.BsonDocument;
 import org.bson.BsonTimestamp;
 import org.bson.BsonValue;
 import org.bson.Document;
-
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
 import org.springframework.data.mongodb.core.query.Collation;
 import org.springframework.lang.Nullable;
@@ -45,7 +42,6 @@ import com.mongodb.client.model.changestream.FullDocument;
  * @author Mark Paluch
  * @since 2.1
  */
-@EqualsAndHashCode
 public class ChangeStreamOptions {
 
 	private @Nullable Object filter;
@@ -156,6 +152,44 @@ public class ChangeStreamOptions {
 						+ ObjectUtils.nullSafeClassName(timestamp));
 	}
 
+	@Override
+	public boolean equals(Object o) {
+		if (this == o)
+			return true;
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		ChangeStreamOptions that = (ChangeStreamOptions) o;
+
+		if (!ObjectUtils.nullSafeEquals(this.filter, that.filter)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(this.resumeToken, that.resumeToken)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(this.fullDocumentLookup, that.fullDocumentLookup)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(this.collation, that.collation)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(this.resumeTimestamp, that.resumeTimestamp)) {
+			return false;
+		}
+		return resume == that.resume;
+	}
+
+	@Override
+	public int hashCode() {
+		int result = ObjectUtils.nullSafeHashCode(filter);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(resumeToken);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(fullDocumentLookup);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(collation);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(resumeTimestamp);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(resume);
+		return result;
+	}
+
 	/**
 	 * @author Christoph Strobl
 	 * @since 2.2

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.RequiredArgsConstructor;
-
 import java.util.Optional;
 
 import org.springframework.data.mongodb.core.query.Collation;
@@ -312,7 +310,6 @@ public class CollectionOptions {
 	 * @author Andreas Zink
 	 * @since 2.1
 	 */
-	@RequiredArgsConstructor
 	public static class ValidationOptions {
 
 		private static final ValidationOptions NONE = new ValidationOptions(null, null, null);
@@ -321,6 +318,13 @@ public class CollectionOptions {
 		private final @Nullable ValidationLevel validationLevel;
 		private final @Nullable ValidationAction validationAction;
 
+		public ValidationOptions(Validator validator, ValidationLevel validationLevel, ValidationAction validationAction) {
+
+			this.validator = validator;
+			this.validationLevel = validationLevel;
+			this.validationAction = validationAction;
+		}
+
 		/**
 		 * Create an empty {@link ValidationOptions}.
 		 *

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

@@ -15,9 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.NonNull;
-import lombok.Value;
-
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
@@ -27,8 +24,9 @@ import java.util.stream.Collectors;
 import org.bson.Document;
 import org.bson.conversions.Bson;
 import org.springframework.context.ApplicationEventPublisher;
-import org.springframework.dao.support.PersistenceExceptionTranslator;
+import org.springframework.dao.DataIntegrityViolationException;
 import org.springframework.data.mapping.callback.EntityCallbacks;
+import org.springframework.data.mongodb.BulkOperationException;
 import org.springframework.data.mongodb.core.convert.QueryMapper;
 import org.springframework.data.mongodb.core.convert.UpdateMapper;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
@@ -47,7 +45,9 @@ import org.springframework.data.mongodb.core.query.UpdateDefinition.ArrayFilter;
 import org.springframework.data.util.Pair;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 
+import com.mongodb.MongoBulkWriteException;
 import com.mongodb.WriteConcern;
 import com.mongodb.bulk.BulkWriteResult;
 import com.mongodb.client.MongoCollection;
@@ -64,6 +64,7 @@ import com.mongodb.client.model.*;
  * @author Jens Schauder
  * @author Michail Nikolaev
  * @author Roman Puchkovskiy
+ * @author Jacob Botuck
  * @since 1.9
  */
 class DefaultBulkOperations implements BulkOperations {
@@ -73,7 +74,6 @@ class DefaultBulkOperations implements BulkOperations {
 	private final BulkOperationContext bulkOperationContext;
 	private final List<SourceAwareWriteModelHolder> models = new ArrayList<>();
 
-	private PersistenceExceptionTranslator exceptionTranslator;
 	private @Nullable WriteConcern defaultWriteConcern;
 
 	private BulkWriteOptions bulkOptions;
@@ -97,19 +97,9 @@ class DefaultBulkOperations implements BulkOperations {
 		this.mongoOperations = mongoOperations;
 		this.collectionName = collectionName;
 		this.bulkOperationContext = bulkOperationContext;
-		this.exceptionTranslator = new MongoExceptionTranslator();
 		this.bulkOptions = getBulkWriteOptions(bulkOperationContext.getBulkMode());
 	}
 
-	/**
-	 * Configures the {@link PersistenceExceptionTranslator} to be used. Defaults to {@link MongoExceptionTranslator}.
-	 *
-	 * @param exceptionTranslator can be {@literal null}.
-	 */
-	public void setExceptionTranslator(@Nullable PersistenceExceptionTranslator exceptionTranslator) {
-		this.exceptionTranslator = exceptionTranslator == null ? new MongoExceptionTranslator() : exceptionTranslator;
-	}
-
 	/**
 	 * Configures the default {@link WriteConcern} to be used. Defaults to {@literal null}.
 	 *
@@ -316,11 +306,26 @@ class DefaultBulkOperations implements BulkOperations {
 			collection = collection.withWriteConcern(defaultWriteConcern);
 		}
 
-		return collection.bulkWrite( //
-				models.stream() //
-						.map(this::extractAndMapWriteModel) //
-						.collect(Collectors.toList()), //
-				bulkOptions);
+		try {
+
+			return collection.bulkWrite( //
+					models.stream() //
+							.map(this::extractAndMapWriteModel) //
+							.collect(Collectors.toList()), //
+					bulkOptions);
+		} catch (RuntimeException ex) {
+
+			if (ex instanceof MongoBulkWriteException) {
+
+				MongoBulkWriteException mongoBulkWriteException = (MongoBulkWriteException) ex;
+				if (mongoBulkWriteException.getWriteConcernError() != null) {
+					throw new DataIntegrityViolationException(ex.getMessage(), ex);
+				}
+				throw new BulkOperationException(ex.getMessage(), mongoBulkWriteException);
+			}
+
+			throw ex;
+		}
 	}
 
 	private WriteModel<Document> extractAndMapWriteModel(SourceAwareWriteModelHolder it) {
@@ -547,15 +552,93 @@ class DefaultBulkOperations implements BulkOperations {
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	@Value
-	static class BulkOperationContext {
+	static final class BulkOperationContext {
 
-		@NonNull BulkMode bulkMode;
-		@NonNull Optional<? extends MongoPersistentEntity<?>> entity;
-		@NonNull QueryMapper queryMapper;
-		@NonNull UpdateMapper updateMapper;
-		ApplicationEventPublisher eventPublisher;
-		EntityCallbacks entityCallbacks;
+		private final BulkMode bulkMode;
+		private final Optional<? extends MongoPersistentEntity<?>> entity;
+		private final QueryMapper queryMapper;
+		private final UpdateMapper updateMapper;
+		private final ApplicationEventPublisher eventPublisher;
+		private final EntityCallbacks entityCallbacks;
+
+		BulkOperationContext(BulkOperations.BulkMode bulkMode, Optional<? extends MongoPersistentEntity<?>> entity,
+				QueryMapper queryMapper, UpdateMapper updateMapper, ApplicationEventPublisher eventPublisher,
+				EntityCallbacks entityCallbacks) {
+
+			this.bulkMode = bulkMode;
+			this.entity = entity;
+			this.queryMapper = queryMapper;
+			this.updateMapper = updateMapper;
+			this.eventPublisher = eventPublisher;
+			this.entityCallbacks = entityCallbacks;
+		}
+
+		public BulkMode getBulkMode() {
+			return this.bulkMode;
+		}
+
+		public Optional<? extends MongoPersistentEntity<?>> getEntity() {
+			return this.entity;
+		}
+
+		public QueryMapper getQueryMapper() {
+			return this.queryMapper;
+		}
+
+		public UpdateMapper getUpdateMapper() {
+			return this.updateMapper;
+		}
+
+		public ApplicationEventPublisher getEventPublisher() {
+			return this.eventPublisher;
+		}
+
+		public EntityCallbacks getEntityCallbacks() {
+			return this.entityCallbacks;
+		}
+
+		@Override
+		public boolean equals(Object o) {
+			if (this == o)
+				return true;
+			if (o == null || getClass() != o.getClass())
+				return false;
+
+			BulkOperationContext that = (BulkOperationContext) o;
+
+			if (bulkMode != that.bulkMode)
+				return false;
+			if (!ObjectUtils.nullSafeEquals(this.entity, that.entity)) {
+				return false;
+			}
+			if (!ObjectUtils.nullSafeEquals(this.queryMapper, that.queryMapper)) {
+				return false;
+			}
+			if (!ObjectUtils.nullSafeEquals(this.updateMapper, that.updateMapper)) {
+				return false;
+			}
+			if (!ObjectUtils.nullSafeEquals(this.eventPublisher, that.eventPublisher)) {
+				return false;
+			}
+			return ObjectUtils.nullSafeEquals(this.entityCallbacks, that.entityCallbacks);
+		}
+
+		@Override
+		public int hashCode() {
+			int result = bulkMode != null ? bulkMode.hashCode() : 0;
+			result = 31 * result + ObjectUtils.nullSafeHashCode(entity);
+			result = 31 * result + ObjectUtils.nullSafeHashCode(queryMapper);
+			result = 31 * result + ObjectUtils.nullSafeHashCode(updateMapper);
+			result = 31 * result + ObjectUtils.nullSafeHashCode(eventPublisher);
+			result = 31 * result + ObjectUtils.nullSafeHashCode(entityCallbacks);
+			return result;
+		}
+
+		public String toString() {
+			return "DefaultBulkOperations.BulkOperationContext(bulkMode=" + this.getBulkMode() + ", entity="
+					+ this.getEntity() + ", queryMapper=" + this.getQueryMapper() + ", updateMapper=" + this.getUpdateMapper()
+					+ ", eventPublisher=" + this.getEventPublisher() + ", entityCallbacks=" + this.getEntityCallbacks() + ")";
+		}
 	}
 
 	/**
@@ -564,10 +647,50 @@ class DefaultBulkOperations implements BulkOperations {
 	 * @since 2.2
 	 * @author Christoph Strobl
 	 */
-	@Value
-	private static class SourceAwareWriteModelHolder {
+	private static final class SourceAwareWriteModelHolder {
 
-		Object source;
-		WriteModel<Document> model;
+		private final Object source;
+		private final WriteModel<Document> model;
+
+		SourceAwareWriteModelHolder(Object source, WriteModel<Document> model) {
+
+			this.source = source;
+			this.model = model;
+		}
+
+		public Object getSource() {
+			return this.source;
+		}
+
+		public WriteModel<Document> getModel() {
+			return this.model;
+		}
+
+		@Override
+		public boolean equals(Object o) {
+			if (this == o)
+				return true;
+			if (o == null || getClass() != o.getClass())
+				return false;
+
+			SourceAwareWriteModelHolder that = (SourceAwareWriteModelHolder) o;
+
+			if (!ObjectUtils.nullSafeEquals(this.source, that.source)) {
+				return false;
+			}
+			return ObjectUtils.nullSafeEquals(this.model, that.model);
+		}
+
+		@Override
+		public int hashCode() {
+			int result = ObjectUtils.nullSafeHashCode(model);
+			result = 31 * result + ObjectUtils.nullSafeHashCode(source);
+			return result;
+		}
+
+		public String toString() {
+			return "DefaultBulkOperations.SourceAwareWriteModelHolder(source=" + this.getSource() + ", model="
+					+ this.getModel() + ")";
+		}
 	}
 }

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

@@ -15,10 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-
 import java.util.Collection;
 import java.util.Map;
 import java.util.Optional;
@@ -55,12 +51,15 @@ import org.springframework.util.MultiValueMap;
  * @see MongoTemplate
  * @see ReactiveMongoTemplate
  */
-@RequiredArgsConstructor
 class EntityOperations {
 
 	private static final String ID_FIELD = "_id";
 
-	private final @NonNull MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> context;
+	private final MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> context;
+
+	EntityOperations(MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> context) {
+		this.context = context;
+	}
 
 	/**
 	 * Creates a new {@link Entity} for the given bean.
@@ -69,7 +68,7 @@ class EntityOperations {
 	 * @return new instance of {@link Entity}.
 	 */
 	@SuppressWarnings({ "unchecked", "rawtypes" })
-	public <T> Entity<T> forEntity(T entity) {
+	<T> Entity<T> forEntity(T entity) {
 
 		Assert.notNull(entity, "Bean must not be null!");
 
@@ -92,7 +91,7 @@ class EntityOperations {
 	 * @return new instance of {@link AdaptibleEntity}.
 	 */
 	@SuppressWarnings({ "unchecked", "rawtypes" })
-	public <T> AdaptibleEntity<T> forEntity(T entity, ConversionService conversionService) {
+	<T> AdaptibleEntity<T> forEntity(T entity, ConversionService conversionService) {
 
 		Assert.notNull(entity, "Bean must not be null!");
 		Assert.notNull(conversionService, "ConversionService must not be null!");
@@ -346,11 +345,14 @@ class EntityOperations {
 		Number getVersion();
 	}
 
-	@RequiredArgsConstructor
 	private static class UnmappedEntity<T extends Map<String, Object>> implements AdaptibleEntity<T> {
 
 		private final T map;
 
+		protected UnmappedEntity(T map) {
+			this.map = map;
+		}
+
 		/*
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.core.EntityOperations.PersistableSource#getIdPropertyName()
@@ -460,7 +462,7 @@ class EntityOperations {
 
 	private static class SimpleMappedEntity<T extends Map<String, Object>> extends UnmappedEntity<T> {
 
-		SimpleMappedEntity(T map) {
+		protected SimpleMappedEntity(T map) {
 			super(map);
 		}
 
@@ -483,12 +485,19 @@ class EntityOperations {
 		}
 	}
 
-	@RequiredArgsConstructor(access = AccessLevel.PROTECTED)
 	private static class MappedEntity<T> implements Entity<T> {
 
-		private final @NonNull MongoPersistentEntity<?> entity;
-		private final @NonNull IdentifierAccessor idAccessor;
-		private final @NonNull PersistentPropertyAccessor<T> propertyAccessor;
+		private final MongoPersistentEntity<?> entity;
+		private final IdentifierAccessor idAccessor;
+		private final PersistentPropertyAccessor<T> propertyAccessor;
+
+		protected MappedEntity(MongoPersistentEntity<?> entity, IdentifierAccessor idAccessor,
+				PersistentPropertyAccessor<T> propertyAccessor) {
+
+			this.entity = entity;
+			this.idAccessor = idAccessor;
+			this.propertyAccessor = propertyAccessor;
+		}
 
 		private static <T> MappedEntity<T> of(T bean,
 				MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> context) {
@@ -759,11 +768,12 @@ class EntityOperations {
 	 * {@link TypedOperations} for generic entities that are not represented with {@link PersistentEntity} (e.g. custom
 	 * conversions).
 	 */
-	@RequiredArgsConstructor
 	enum UntypedOperations implements TypedOperations<Object> {
 
 		INSTANCE;
 
+		UntypedOperations() {}
+
 		@SuppressWarnings({ "unchecked", "rawtypes" })
 		public static <T> TypedOperations<T> instance() {
 			return (TypedOperations) INSTANCE;
@@ -798,10 +808,13 @@ class EntityOperations {
 	 *
 	 * @param <T>
 	 */
-	@RequiredArgsConstructor
 	static class TypedEntityOperations<T> implements TypedOperations<T> {
 
-		private final @NonNull MongoPersistentEntity<T> entity;
+		private final MongoPersistentEntity<T> entity;
+
+		protected TypedEntityOperations(MongoPersistentEntity<T> entity) {
+			this.entity = entity;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,16 +15,10 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
-
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
 import org.springframework.data.mongodb.core.aggregation.AggregationResults;
 import org.springframework.data.mongodb.core.aggregation.TypedAggregation;
 import org.springframework.data.util.CloseableIterator;
-import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
 
@@ -35,10 +29,13 @@ import org.springframework.util.StringUtils;
  * @author Mark Paluch
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ExecutableAggregationOperationSupport implements ExecutableAggregationOperation {
 
-	private final @NonNull MongoTemplate template;
+	private final MongoTemplate template;
+
+	ExecutableAggregationOperationSupport(MongoTemplate template) {
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -56,15 +53,21 @@ class ExecutableAggregationOperationSupport implements ExecutableAggregationOper
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ExecutableAggregationSupport<T>
 			implements AggregationWithAggregation<T>, ExecutableAggregation<T>, TerminatingAggregation<T> {
 
-		@NonNull MongoTemplate template;
-		@NonNull Class<T> domainType;
-		@Nullable Aggregation aggregation;
-		@Nullable String collection;
+		private final MongoTemplate template;
+		private final Class<T> domainType;
+		private final Aggregation aggregation;
+		private final String collection;
+
+		public ExecutableAggregationSupport(MongoTemplate template, Class<T> domainType, Aggregation aggregation,
+				String collection) {
+			this.template = template;
+			this.domainType = domainType;
+			this.aggregation = aggregation;
+			this.collection = collection;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,12 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import com.mongodb.ReadPreference;
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
-
 import java.util.List;
 import java.util.Optional;
 import java.util.stream.Stream;
@@ -37,6 +31,7 @@ import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 
+import com.mongodb.ReadPreference;
 import com.mongodb.client.FindIterable;
 
 /**
@@ -46,12 +41,15 @@ import com.mongodb.client.FindIterable;
  * @author Mark Paluch
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ExecutableFindOperationSupport implements ExecutableFindOperation {
 
 	private static final Query ALL_QUERY = new Query();
 
-	private final @NonNull MongoTemplate template;
+	private final MongoTemplate template;
+
+	ExecutableFindOperationSupport(MongoTemplate template) {
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -70,16 +68,23 @@ class ExecutableFindOperationSupport implements ExecutableFindOperation {
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ExecutableFindSupport<T>
 			implements ExecutableFind<T>, FindWithCollection<T>, FindWithProjection<T>, FindWithQuery<T> {
 
-		@NonNull MongoTemplate template;
-		@NonNull Class<?> domainType;
-		Class<T> returnType;
-		@Nullable String collection;
-		Query query;
+		private final MongoTemplate template;
+		private final Class<?> domainType;
+		private final Class<T> returnType;
+		@Nullable private final String collection;
+		private final Query query;
+
+		ExecutableFindSupport(MongoTemplate template, Class<?> domainType, Class<T> returnType,
+				String collection, Query query) {
+			this.template = template;
+			this.domainType = domainType;
+			this.returnType = returnType;
+			this.collection = collection;
+			this.query = query;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,11 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
-
 import java.util.ArrayList;
 import java.util.Collection;
 
@@ -37,10 +32,13 @@ import com.mongodb.bulk.BulkWriteResult;
  * @author Mark Paluch
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ExecutableInsertOperationSupport implements ExecutableInsertOperation {
 
-	private final @NonNull MongoTemplate template;
+	private final MongoTemplate template;
+
+	ExecutableInsertOperationSupport(MongoTemplate template) {
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -58,14 +56,20 @@ class ExecutableInsertOperationSupport implements ExecutableInsertOperation {
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ExecutableInsertSupport<T> implements ExecutableInsert<T> {
 
-		@NonNull MongoTemplate template;
-		@NonNull Class<T> domainType;
-		@Nullable String collection;
-		@Nullable BulkMode bulkMode;
+		private final MongoTemplate template;
+		private final Class<T> domainType;
+		@Nullable private final String collection;
+		@Nullable private final BulkMode bulkMode;
+
+		ExecutableInsertSupport(MongoTemplate template, Class<T> domainType, String collection, BulkMode bulkMode) {
+
+			this.template = template;
+			this.domainType = domainType;
+			this.collection = collection;
+			this.bulkMode = bulkMode;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,9 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-
 import java.util.List;
 
 import org.springframework.data.mongodb.core.mapreduce.MapReduceOptions;
@@ -32,12 +29,17 @@ import org.springframework.util.StringUtils;
  * @author Christoph Strobl
  * @since 2.1
  */
-@RequiredArgsConstructor
 class ExecutableMapReduceOperationSupport implements ExecutableMapReduceOperation {
 
 	private static final Query ALL_QUERY = new Query();
 
-	private final @NonNull MongoTemplate template;
+	private final MongoTemplate template;
+
+	ExecutableMapReduceOperationSupport(MongoTemplate template) {
+		
+		Assert.notNull(template, "Template must not be null!");
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javascript)

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

@@ -15,11 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
-
 import java.util.List;
 
 import org.springframework.data.mongodb.core.query.Query;
@@ -36,12 +31,15 @@ import com.mongodb.client.result.DeleteResult;
  * @author Mark Paluch
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ExecutableRemoveOperationSupport implements ExecutableRemoveOperation {
 
 	private static final Query ALL_QUERY = new Query();
 
-	private final @NonNull MongoTemplate tempate;
+	private final MongoTemplate tempate;
+
+	public ExecutableRemoveOperationSupport(MongoTemplate tempate) {
+		this.tempate = tempate;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -59,14 +57,19 @@ class ExecutableRemoveOperationSupport implements ExecutableRemoveOperation {
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ExecutableRemoveSupport<T> implements ExecutableRemove<T>, RemoveWithCollection<T> {
 
-		@NonNull MongoTemplate template;
-		@NonNull Class<T> domainType;
-		Query query;
-		@Nullable String collection;
+		private final MongoTemplate template;
+		private final Class<T> domainType;
+		private final Query query;
+		@Nullable private final String collection;
+
+		public ExecutableRemoveSupport(MongoTemplate template, Class<T> domainType, Query query, String collection) {
+			this.template = template;
+			this.domainType = domainType;
+			this.query = query;
+			this.collection = collection;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,11 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
-
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.lang.Nullable;
@@ -35,12 +30,15 @@ import com.mongodb.client.result.UpdateResult;
  * @author Mark Paluch
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ExecutableUpdateOperationSupport implements ExecutableUpdateOperation {
 
 	private static final Query ALL_QUERY = new Query();
 
-	private final @NonNull MongoTemplate template;
+	private final MongoTemplate template;
+
+	ExecutableUpdateOperationSupport(MongoTemplate template) {
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -58,21 +56,34 @@ class ExecutableUpdateOperationSupport implements ExecutableUpdateOperation {
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ExecutableUpdateSupport<T>
 			implements ExecutableUpdate<T>, UpdateWithCollection<T>, UpdateWithQuery<T>, TerminatingUpdate<T>,
 			FindAndReplaceWithOptions<T>, TerminatingFindAndReplace<T>, FindAndReplaceWithProjection<T> {
 
-		@NonNull MongoTemplate template;
-		@NonNull Class domainType;
-		Query query;
-		@Nullable UpdateDefinition update;
-		@Nullable String collection;
-		@Nullable FindAndModifyOptions findAndModifyOptions;
-		@Nullable FindAndReplaceOptions findAndReplaceOptions;
-		@Nullable Object replacement;
-		@NonNull Class<T> targetType;
+		private final MongoTemplate template;
+		private final Class domainType;
+		private final Query query;
+		@Nullable private final UpdateDefinition update;
+		@Nullable private final String collection;
+		@Nullable private final FindAndModifyOptions findAndModifyOptions;
+		@Nullable private final FindAndReplaceOptions findAndReplaceOptions;
+		@Nullable private final Object replacement;
+		private final Class<T> targetType;
+
+		ExecutableUpdateSupport(MongoTemplate template, Class domainType, Query query, UpdateDefinition update,
+				String collection, FindAndModifyOptions findAndModifyOptions, FindAndReplaceOptions findAndReplaceOptions,
+				Object replacement, Class<T> targetType) {
+
+			this.template = template;
+			this.domainType = domainType;
+			this.query = query;
+			this.update = update;
+			this.collection = collection;
+			this.findAndModifyOptions = findAndModifyOptions;
+			this.findAndReplaceOptions = findAndReplaceOptions;
+			this.replacement = replacement;
+			this.targetType = targetType;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,9 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.Getter;
-import lombok.RequiredArgsConstructor;
-
 import java.util.Collection;
 import java.util.List;
 
@@ -27,8 +24,6 @@ import org.springframework.data.mongodb.core.query.Update;
 import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.data.util.StreamUtils;
 
-import com.mongodb.client.model.Filters;
-
 /**
  * A MongoDB document in its mapped state. I.e. after a source document has been mapped using mapping information of the
  * entity the source document was supposed to represent.
@@ -36,13 +31,20 @@ import com.mongodb.client.model.Filters;
  * @author Oliver Gierke
  * @since 2.1
  */
-@RequiredArgsConstructor(staticName = "of")
 public class MappedDocument {
 
 	private static final String ID_FIELD = "_id";
 	private static final Document ID_ONLY_PROJECTION = new Document(ID_FIELD, 1);
 
-	private final @Getter Document document;
+	private final Document document;
+
+	private MappedDocument(Document document) {
+		this.document = document;
+	}
+
+	public static MappedDocument of(Document document) {
+		return new MappedDocument(document);
+	}
 
 	public static Document getIdOnlyProjection() {
 		return ID_ONLY_PROJECTION;
@@ -91,6 +93,10 @@ public class MappedDocument {
 		return new MappedUpdate(Update.fromDocument(document, ID_FIELD));
 	}
 
+	public Document getDocument() {
+		return this.document;
+	}
+
 	/**
 	 * An {@link UpdateDefinition} that indicates that the {@link #getUpdateObject() update object} has already been
 	 * mapped to the specific domain type.

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.Value;
-
 import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.dao.DataAccessException;
 import org.springframework.dao.support.PersistenceExceptionTranslator;
@@ -24,6 +22,7 @@ import org.springframework.data.mongodb.MongoDatabaseFactory;
 import org.springframework.data.mongodb.SessionAwareMethodInterceptor;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 
 import com.mongodb.ClientSessionOptions;
 import com.mongodb.WriteConcern;
@@ -171,11 +170,15 @@ public abstract class MongoDatabaseFactorySupport<C> implements MongoDatabaseFac
 	 * @author Christoph Strobl
 	 * @since 2.1
 	 */
-	@Value
-	static class ClientSessionBoundMongoDbFactory implements MongoDatabaseFactory {
+	static final class ClientSessionBoundMongoDbFactory implements MongoDatabaseFactory {
 
-		ClientSession session;
-		MongoDatabaseFactory delegate;
+		private final ClientSession session;
+		private final MongoDatabaseFactory delegate;
+
+		public ClientSessionBoundMongoDbFactory(ClientSession session, MongoDatabaseFactory delegate) {
+			this.session = session;
+			this.delegate = delegate;
+		}
 
 		/*
 		 * (non-Javadoc)
@@ -254,7 +257,42 @@ 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()));
+		}
+
+		public ClientSession getSession() {
+			return this.session;
+		}
+
+		public MongoDatabaseFactory getDelegate() {
+			return this.delegate;
+		}
+
+		@Override
+		public boolean equals(Object o) {
+			if (this == o)
+				return true;
+			if (o == null || getClass() != o.getClass())
+				return false;
+
+			ClientSessionBoundMongoDbFactory that = (ClientSessionBoundMongoDbFactory) o;
+
+			if (!ObjectUtils.nullSafeEquals(this.session, that.session)) {
+				return false;
+			}
+			return ObjectUtils.nullSafeEquals(this.delegate, that.delegate);
+		}
+
+		@Override
+		public int hashCode() {
+			int result = ObjectUtils.nullSafeHashCode(this.session);
+			result = 31 * result + ObjectUtils.nullSafeHashCode(this.delegate);
+			return result;
+		}
+
+		public String toString() {
+			return "MongoDatabaseFactorySupport.ClientSessionBoundMongoDbFactory(session=" + this.getSession() + ", delegate="
+					+ this.getDelegate() + ")";
 		}
 	}
 }

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

@@ -1184,6 +1184,29 @@ public interface MongoOperations extends FluentMongoOperations {
 	 */
 	long count(Query query, String collectionName);
 
+	/**
+	 * Estimate the number of documents, in the collection {@link #getCollectionName(Class) identified by the given type},
+	 * based on collection statistics.
+	 *
+	 * @param entityClass must not be {@literal null}.
+	 * @return the estimated number of documents.
+	 * @since 3.1
+	 */
+	default long estimatedCount(Class<?> entityClass) {
+
+		Assert.notNull(entityClass, "Entity class must not be null!");
+		return estimatedCount(getCollectionName(entityClass));
+	}
+
+	/**
+	 * Estimate the number of documents in the given collection based on collection statistics.
+	 *
+	 * @param collectionName must not be {@literal null}.
+	 * @return the estimated number of documents.
+	 * @since 3.1
+	 */
+	long estimatedCount(String collectionName);
+
 	/**
 	 * Returns the number of documents for the given {@link Query} by querying the given collection using the given entity
 	 * class to map the given {@link Query}. <br />

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

@@ -17,11 +17,6 @@ package org.springframework.data.mongodb.core;
 
 import static org.springframework.data.mongodb.core.query.SerializationUtils.*;
 
-import lombok.AccessLevel;
-import lombok.AllArgsConstructor;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-
 import java.io.IOException;
 import java.math.BigDecimal;
 import java.math.RoundingMode;
@@ -160,6 +155,7 @@ import com.mongodb.client.result.UpdateResult;
  * @author Cimon Lucas
  * @author Michael J. Simons
  * @author Roman Puchkovskiy
+ * @author Yadhukrishna S Pai
  */
 public class MongoTemplate implements MongoOperations, ApplicationContextAware, IndexOperationsProvider {
 
@@ -763,7 +759,6 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 				new BulkOperationContext(mode, Optional.ofNullable(getPersistentEntity(entityType)), queryMapper, updateMapper,
 						eventPublisher, entityCallbacks));
 
-		operations.setExceptionTranslator(exceptionTranslator);
 		operations.setDefaultWriteConcern(writeConcern);
 
 		return operations;
@@ -1140,6 +1135,19 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 				collection -> collection.countDocuments(CountQuery.of(filter).toQueryDocument(), options));
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.MongoOperations#estimatedCount(java.lang.String)
+	 */
+	@Override
+	public long estimatedCount(String collectionName) {
+		return doEstimatedCount(collectionName, new EstimatedDocumentCountOptions());
+	}
+
+	protected long doEstimatedCount(String collectionName, EstimatedDocumentCountOptions options) {
+		return execute(collectionName, collection -> collection.estimatedDocumentCount(options));
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.MongoOperations#insert(java.lang.Object)
@@ -1178,7 +1186,7 @@ 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
 	 */
@@ -1969,9 +1977,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 
 		Assert.notNull(aggregation, "Aggregation pipeline must not be null!");
 
-		AggregationOperationContext context = new TypeBasedAggregationOperationContext(aggregation.getInputType(),
-				mappingContext, queryMapper);
-		return aggregate(aggregation, inputCollectionName, outputType, context);
+		return aggregate(aggregation, inputCollectionName, outputType, null);
 	}
 
 	/* (non-Javadoc)
@@ -2125,7 +2131,7 @@ 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,
@@ -2141,11 +2147,23 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			}
 
 			options.getComment().ifPresent(aggregateIterable::comment);
+			options.getHint().ifPresent(aggregateIterable::hint);
 
 			if (options.hasExecutionTimeLimit()) {
 				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);
@@ -2188,6 +2206,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 			}
 
 			options.getComment().ifPresent(cursor::comment);
+			options.getHint().ifPresent(cursor::hint);
 
 			Class<?> domainType = aggregation instanceof TypedAggregation ? ((TypedAggregation) aggregation).getInputType()
 					: null;
@@ -2950,12 +2969,17 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
 	private class ExistsCallback implements CollectionCallback<Boolean> {
 
 		private final Document mappedQuery;
 		private final com.mongodb.client.model.Collation collation;
 
+		ExistsCallback(Document mappedQuery, com.mongodb.client.model.Collation collation) {
+
+			this.mappedQuery = mappedQuery;
+			this.collation = collation;
+		}
+
 		@Override
 		public Boolean doInCollection(MongoCollection<Document> collection) throws MongoException, DataAccessException {
 
@@ -2977,7 +3001,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		private final Document sort;
 		private final Optional<Collation> collation;
 
-		public FindAndRemoveCallback(Document query, Document fields, Document sort, @Nullable Collation collation) {
+		FindAndRemoveCallback(Document query, Document fields, Document sort, @Nullable Collation collation) {
 
 			this.query = query;
 			this.fields = fields;
@@ -3003,8 +3027,9 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		private final List<Document> arrayFilters;
 		private final FindAndModifyOptions options;
 
-		public FindAndModifyCallback(Document query, Document fields, Document sort, Object update,
-				List<Document> arrayFilters, FindAndModifyOptions options) {
+		FindAndModifyCallback(Document query, Document fields, Document sort, Object update, List<Document> arrayFilters,
+				FindAndModifyOptions options) {
+
 			this.query = query;
 			this.fields = fields;
 			this.sort = sort;
@@ -3113,13 +3138,19 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @author Christoph Strobl
 	 * @author Roman Puchkovskiy
 	 */
-	@RequiredArgsConstructor
 	private class ReadDocumentCallback<T> implements DocumentCallback<T> {
 
-		private final @NonNull EntityReader<? super T, Bson> reader;
-		private final @NonNull Class<T> type;
+		private final EntityReader<? super T, Bson> reader;
+		private final Class<T> type;
 		private final String collectionName;
 
+		ReadDocumentCallback(EntityReader<? super T, Bson> reader, Class<T> type, String collectionName) {
+
+			this.reader = reader;
+			this.type = type;
+			this.collectionName = collectionName;
+		}
+
 		@Nullable
 		public T doWith(@Nullable Document document) {
 
@@ -3147,13 +3178,21 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @param <T>
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
 	private class ProjectingReadCallback<S, T> implements DocumentCallback<T> {
 
-		private final @NonNull EntityReader<Object, Bson> reader;
-		private final @NonNull Class<S> entityType;
-		private final @NonNull Class<T> targetType;
-		private final @NonNull String collectionName;
+		private final EntityReader<Object, Bson> reader;
+		private final Class<S> entityType;
+		private final Class<T> targetType;
+		private final String collectionName;
+
+		ProjectingReadCallback(EntityReader<Object, Bson> reader, Class<S> entityType, Class<T> targetType,
+				String collectionName) {
+
+			this.reader = reader;
+			this.entityType = entityType;
+			this.targetType = targetType;
+			this.collectionName = collectionName;
+		}
 
 		/*
 		 * (non-Javadoc)
@@ -3189,7 +3228,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		private final Query query;
 		private final @Nullable Class<?> type;
 
-		public QueryCursorPreparer(Query query, @Nullable Class<?> type) {
+		QueryCursorPreparer(Query query, @Nullable Class<?> type) {
 
 			this.query = query;
 			this.type = type;
@@ -3260,6 +3299,7 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 							case PARTIAL:
 								cursorToUse = cursorToUse.partial(true);
 								break;
+							case SECONDARY_READS:
 							case SLAVE_OK:
 								break;
 							default:
@@ -3277,7 +3317,8 @@ 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;
 		}
 	}
 
@@ -3331,7 +3372,6 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 	 * @author Thomas Darimont
 	 * @since 1.7
 	 */
-	@AllArgsConstructor(access = AccessLevel.PACKAGE)
 	static class CloseableIterableCursorAdapter<T> implements CloseableIterator<T> {
 
 		private volatile @Nullable MongoCursor<Document> cursor;
@@ -3345,14 +3385,22 @@ public class MongoTemplate implements MongoOperations, ApplicationContextAware,
 		 * @param exceptionTranslator
 		 * @param objectReadCallback
 		 */
-		public CloseableIterableCursorAdapter(MongoIterable<Document> cursor,
-				PersistenceExceptionTranslator exceptionTranslator, DocumentCallback<T> objectReadCallback) {
+		CloseableIterableCursorAdapter(MongoIterable<Document> cursor, PersistenceExceptionTranslator exceptionTranslator,
+				DocumentCallback<T> objectReadCallback) {
 
 			this.cursor = cursor.iterator();
 			this.exceptionTranslator = exceptionTranslator;
 			this.objectReadCallback = objectReadCallback;
 		}
 
+		CloseableIterableCursorAdapter(MongoCursor<Document> cursor, PersistenceExceptionTranslator exceptionTranslator,
+				DocumentCallback<T> objectReadCallback) {
+
+			this.cursor = cursor;
+			this.exceptionTranslator = exceptionTranslator;
+			this.objectReadCallback = objectReadCallback;
+		}
+
 		@Override
 		public boolean hasNext() {
 

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

@@ -15,9 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.RequiredArgsConstructor;
-
 import org.bson.Document;
 import org.springframework.data.mapping.SimplePropertyHandler;
 import org.springframework.data.mapping.context.MappingContext;
@@ -33,11 +30,14 @@ import org.springframework.util.ClassUtils;
  * @author Christoph Strobl
  * @since 2.1
  */
-@RequiredArgsConstructor(access = AccessLevel.PACKAGE)
 class PropertyOperations {
 
 	private final MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext;
 
+	PropertyOperations(MappingContext<? extends MongoPersistentEntity<?>, MongoPersistentProperty> mappingContext) {
+		this.mappingContext = mappingContext;
+	}
+
 	/**
 	 * For cases where {@code fields} is {@link Document#isEmpty() empty} include only fields that are required for
 	 * creating the projection (target) type if the {@code targetType} is a {@literal DTO projection} or a

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

@@ -15,10 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
 import reactor.core.publisher.Flux;
 
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
@@ -62,15 +58,22 @@ class ReactiveAggregationOperationSupport implements ReactiveAggregationOperatio
 		return new ReactiveAggregationSupport<>(template, domainType, null, null);
 	}
 
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ReactiveAggregationSupport<T>
 			implements AggregationOperationWithAggregation<T>, ReactiveAggregation<T>, TerminatingAggregationOperation<T> {
 
-		@NonNull ReactiveMongoTemplate template;
-		@NonNull Class<T> domainType;
-		Aggregation aggregation;
-		String collection;
+		private final ReactiveMongoTemplate template;
+		private final Class<T> domainType;
+		private final Aggregation aggregation;
+		private final String collection;
+
+		ReactiveAggregationSupport(ReactiveMongoTemplate template, Class<T> domainType, Aggregation aggregation,
+				String collection) {
+
+			this.template = template;
+			this.domainType = domainType;
+			this.aggregation = aggregation;
+			this.collection = collection;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,15 +15,10 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.bson.Document;
-
 import org.springframework.dao.IncorrectResultSizeDataAccessException;
 import org.springframework.data.mongodb.core.query.NearQuery;
 import org.springframework.data.mongodb.core.query.Query;
@@ -39,12 +34,15 @@ import org.springframework.util.StringUtils;
  * @author Christoph Strobl
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ReactiveFindOperationSupport implements ReactiveFindOperation {
 
 	private static final Query ALL_QUERY = new Query();
 
-	private final @NonNull ReactiveMongoTemplate template;
+	private final ReactiveMongoTemplate template;
+
+	ReactiveFindOperationSupport(ReactiveMongoTemplate template) {
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -64,16 +62,24 @@ class ReactiveFindOperationSupport implements ReactiveFindOperation {
 	 * @author Christoph Strobl
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ReactiveFindSupport<T>
 			implements ReactiveFind<T>, FindWithCollection<T>, FindWithProjection<T>, FindWithQuery<T> {
 
-		@NonNull ReactiveMongoTemplate template;
-		@NonNull Class<?> domainType;
-		Class<T> returnType;
-		String collection;
-		Query query;
+		private final ReactiveMongoTemplate template;
+		private final Class<?> domainType;
+		private final Class<T> returnType;
+		private final String collection;
+		private final Query query;
+
+		ReactiveFindSupport(ReactiveMongoTemplate template, Class<?> domainType, Class<T> returnType,
+				String collection, Query query) {
+
+			this.template = template;
+			this.domainType = domainType;
+			this.returnType = returnType;
+			this.collection = collection;
+			this.query = query;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,10 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
@@ -34,10 +30,13 @@ import org.springframework.util.StringUtils;
  * @author Christoph Strobl
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ReactiveInsertOperationSupport implements ReactiveInsertOperation {
 
-	private final @NonNull ReactiveMongoTemplate template;
+	private final ReactiveMongoTemplate template;
+
+	ReactiveInsertOperationSupport(ReactiveMongoTemplate template) {
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -51,13 +50,18 @@ class ReactiveInsertOperationSupport implements ReactiveInsertOperation {
 		return new ReactiveInsertSupport<>(template, domainType, null);
 	}
 
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ReactiveInsertSupport<T> implements ReactiveInsert<T> {
 
-		@NonNull ReactiveMongoTemplate template;
-		@NonNull Class<T> domainType;
-		String collection;
+		private final ReactiveMongoTemplate template;
+		private final Class<T> domainType;
+		private final String collection;
+
+		ReactiveInsertSupport(ReactiveMongoTemplate template, Class<T> domainType, String collection) {
+
+			this.template = template;
+			this.domainType = domainType;
+			this.collection = collection;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
 import reactor.core.publisher.Flux;
 
 import org.springframework.data.mongodb.core.mapreduce.MapReduceOptions;
@@ -31,12 +29,15 @@ import org.springframework.util.StringUtils;
  * @author Christoph Strobl
  * @since 2.1
  */
-@RequiredArgsConstructor
 class ReactiveMapReduceOperationSupport implements ReactiveMapReduceOperation {
 
 	private static final Query ALL_QUERY = new Query();
 
-	private final @NonNull ReactiveMongoTemplate template;
+	private final ReactiveMongoTemplate template;
+
+	ReactiveMapReduceOperationSupport(ReactiveMongoTemplate template) {
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javascript)

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

@@ -15,11 +15,15 @@
  */
 package org.springframework.data.mongodb.core;
 
-import org.reactivestreams.Publisher;
-import org.springframework.util.Assert;
 import reactor.core.publisher.Mono;
 import reactor.util.context.Context;
 
+import java.util.function.Function;
+
+import org.reactivestreams.Publisher;
+
+import org.springframework.util.Assert;
+
 import com.mongodb.reactivestreams.client.ClientSession;
 
 /**
@@ -29,7 +33,7 @@ import com.mongodb.reactivestreams.client.ClientSession;
  * @author Christoph Strobl
  * @author Mark Paluch
  * @since 2.1
- * @see Mono#subscriberContext()
+ * @see Mono#deferContextual(Function)
  * @see Context
  */
 public class ReactiveMongoContext {
@@ -46,8 +50,14 @@ public class ReactiveMongoContext {
 	 */
 	public static Mono<ClientSession> getSession() {
 
-		return Mono.subscriberContext().filter(ctx -> ctx.hasKey(SESSION_KEY))
-				.flatMap(ctx -> ctx.<Mono<ClientSession>> get(SESSION_KEY));
+		return Mono.deferContextual(ctx -> {
+
+			if (ctx.hasKey(SESSION_KEY)) {
+				return ctx.<Mono<ClientSession>> get(SESSION_KEY);
+			}
+
+			return Mono.empty();
+		});
 	}
 
 	/**

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

@@ -980,6 +980,29 @@ public interface ReactiveMongoOperations extends ReactiveFluentMongoOperations {
 	 */
 	Mono<Long> count(Query query, @Nullable Class<?> entityClass, String collectionName);
 
+	/**
+	 * Estimate the number of documents, in the collection {@link #getCollectionName(Class) identified by the given type},
+	 * based on collection statistics.
+	 *
+	 * @param entityClass must not be {@literal null}.
+	 * @return a {@link Mono} emitting the estimated number of documents.
+	 * @since 3.1
+	 */
+	default Mono<Long> estimatedCount(Class<?> entityClass) {
+
+		Assert.notNull(entityClass, "Entity class must not be null!");
+		return estimatedCount(getCollectionName(entityClass));
+	}
+
+	/**
+	 * Estimate the number of documents in the given collection based on collection statistics.
+	 *
+	 * @param collectionName must not be {@literal null}.
+	 * @return a {@link Mono} emitting the estimated number of documents.
+	 * @since 3.1
+	 */
+	Mono<Long> estimatedCount(String collectionName);
+
 	/**
 	 * Insert the object into the collection for the entity type of the object to save.
 	 * <p/>

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

@@ -17,9 +17,6 @@ package org.springframework.data.mongodb.core;
 
 import static org.springframework.data.mongodb.core.query.SerializationUtils.*;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.util.function.Tuple2;
@@ -39,6 +36,7 @@ import org.reactivestreams.Publisher;
 import org.reactivestreams.Subscriber;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
+
 import org.springframework.beans.BeansException;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextAware;
@@ -119,16 +117,7 @@ import com.mongodb.CursorType;
 import com.mongodb.MongoException;
 import com.mongodb.ReadPreference;
 import com.mongodb.WriteConcern;
-import com.mongodb.client.model.CountOptions;
-import com.mongodb.client.model.CreateCollectionOptions;
-import com.mongodb.client.model.DeleteOptions;
-import com.mongodb.client.model.FindOneAndDeleteOptions;
-import com.mongodb.client.model.FindOneAndReplaceOptions;
-import com.mongodb.client.model.FindOneAndUpdateOptions;
-import com.mongodb.client.model.ReplaceOptions;
-import com.mongodb.client.model.ReturnDocument;
-import com.mongodb.client.model.UpdateOptions;
-import com.mongodb.client.model.ValidationOptions;
+import com.mongodb.client.model.*;
 import com.mongodb.client.model.changestream.FullDocument;
 import com.mongodb.client.result.DeleteResult;
 import com.mongodb.client.result.InsertOneResult;
@@ -157,6 +146,7 @@ import com.mongodb.reactivestreams.client.MongoDatabase;
  * @author Christoph Strobl
  * @author Roman Puchkovskiy
  * @author Mathieu Ouellet
+ * @author Yadhukrishna S Pai
  * @since 2.0
  */
 public class ReactiveMongoTemplate implements ReactiveMongoOperations, ApplicationContextAware {
@@ -583,7 +573,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 				ReactiveMongoTemplate.this);
 
 		return Flux.from(action.doInSession(operations)) //
-				.subscriberContext(ctx -> ReactiveMongoContext.setSession(ctx, Mono.just(session)));
+				.contextWrite(ctx -> ReactiveMongoContext.setSession(ctx, Mono.just(session)));
 	}
 
 	/*
@@ -1016,11 +1006,14 @@ 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)
@@ -1031,6 +1024,7 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		}
 
 		options.getComment().ifPresent(cursor::comment);
+		options.getHint().ifPresent(cursor::hint);
 
 		Optionals.firstNonEmpty(options::getCollation, () -> operations.forType(inputType).getCollation()) //
 				.map(Collation::toMongoCollation) //
@@ -1040,6 +1034,10 @@ 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);
 	}
 
@@ -1250,6 +1248,15 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		});
 	}
 
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#estimatedCount(java.lang.String)
+	 */
+	@Override
+	public Mono<Long> estimatedCount(String collectionName) {
+		return doEstimatedCount(collectionName, new EstimatedDocumentCountOptions());
+	}
+
 	/**
 	 * Run the actual count operation against the collection with given name.
 	 *
@@ -1264,6 +1271,11 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 				collection -> collection.countDocuments(CountQuery.of(filter).toQueryDocument(), options));
 	}
 
+	protected Mono<Long> doEstimatedCount(String collectionName, EstimatedDocumentCountOptions options) {
+
+		return createMono(collectionName, collection -> collection.estimatedDocumentCount(options));
+	}
+
 	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.ReactiveMongoOperations#insert(reactor.core.publisher.Mono)
@@ -2410,8 +2422,8 @@ 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 com.mongodb.client.FindIterable} used when iterating over the result set, (apply
-	 *          limits, skips and so on).
+	 * @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.
 	 */
 	protected <T> Flux<T> doFind(String collectionName, Document query, Document fields, Class<T> entityClass,
@@ -2682,7 +2694,7 @@ 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
 	 */
@@ -2885,7 +2897,6 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 *
 	 * @author Mark Paluch
 	 */
-	@RequiredArgsConstructor
 	private static class FindCallback implements ReactiveCollectionQueryCallback<Document> {
 
 		private final @Nullable Document query;
@@ -2895,6 +2906,12 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 			this(query, null);
 		}
 
+		FindCallback(Document query, Document fields) {
+
+			this.query = query;
+			this.fields = fields;
+		}
+
 		@Override
 		public FindPublisher<Document> doInCollection(MongoCollection<Document> collection) {
 
@@ -2948,7 +2965,6 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	/**
 	 * @author Mark Paluch
 	 */
-	@RequiredArgsConstructor
 	private static class FindAndModifyCallback implements ReactiveCollectionCallback<Document> {
 
 		private final Document query;
@@ -2958,6 +2974,17 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		private final List<Document> arrayFilters;
 		private final FindAndModifyOptions options;
 
+		FindAndModifyCallback(Document query, Document fields, Document sort, Object update, List<Document> arrayFilters,
+				FindAndModifyOptions options) {
+
+			this.query = query;
+			this.fields = fields;
+			this.sort = sort;
+			this.update = update;
+			this.arrayFilters = arrayFilters;
+			this.options = options;
+		}
+
 		@Override
 		public Publisher<Document> doInCollection(MongoCollection<Document> collection)
 				throws MongoException, DataAccessException {
@@ -3013,7 +3040,6 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * @author Christoph Strobl
 	 * @since 2.1
 	 */
-	@RequiredArgsConstructor(access = AccessLevel.PACKAGE)
 	private static class FindAndReplaceCallback implements ReactiveCollectionCallback<Document> {
 
 		private final Document query;
@@ -3023,6 +3049,17 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		private final @Nullable com.mongodb.client.model.Collation collation;
 		private final FindAndReplaceOptions options;
 
+		FindAndReplaceCallback(Document query, Document fields, Document sort, Document update,
+				com.mongodb.client.model.Collation collation, FindAndReplaceOptions options) {
+
+			this.query = query;
+			this.fields = fields;
+			this.sort = sort;
+			this.update = update;
+			this.collation = collation;
+			this.options = options;
+		}
+
 		/*
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.core.ReactiveCollectionCallback#doInCollection(com.mongodb.reactivestreams.client.MongoCollection)
@@ -3139,13 +3176,20 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 	 * @author Roman Puchkovskiy
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor
 	private class ProjectingReadCallback<S, T> implements DocumentCallback<T> {
 
-		private final @NonNull EntityReader<Object, Bson> reader;
-		private final @NonNull Class<S> entityType;
-		private final @NonNull Class<T> targetType;
-		private final @NonNull String collectionName;
+		private final EntityReader<Object, Bson> reader;
+		private final Class<S> entityType;
+		private final Class<T> targetType;
+		private final String collectionName;
+
+		ProjectingReadCallback(EntityReader<Object, Bson> reader, Class<S> entityType, Class<T> targetType,
+				String collectionName) {
+			this.reader = reader;
+			this.entityType = entityType;
+			this.targetType = targetType;
+			this.collectionName = collectionName;
+		}
 
 		@SuppressWarnings("unchecked")
 		public Mono<T> doWith(Document document) {
@@ -3295,7 +3339,8 @@ 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;
 		}
 	}
 
@@ -3365,11 +3410,14 @@ public class ReactiveMongoTemplate implements ReactiveMongoOperations, Applicati
 		}
 	}
 
-	@RequiredArgsConstructor
 	class IndexCreatorEventListener implements ApplicationListener<MappingContextEvent<?, ?>> {
 
 		final Consumer<Throwable> subscriptionExceptionHandler;
 
+		public IndexCreatorEventListener(Consumer<Throwable> subscriptionExceptionHandler) {
+			this.subscriptionExceptionHandler = subscriptionExceptionHandler;
+		}
+
 		@Override
 		public void onApplicationEvent(MappingContextEvent<?, ?> event) {
 

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

@@ -15,10 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
@@ -35,12 +31,15 @@ import com.mongodb.client.result.DeleteResult;
  * @author Christoph Strobl
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ReactiveRemoveOperationSupport implements ReactiveRemoveOperation {
 
 	private static final Query ALL_QUERY = new Query();
 
-	private final @NonNull ReactiveMongoTemplate tempate;
+	private final ReactiveMongoTemplate tempate;
+
+	ReactiveRemoveOperationSupport(ReactiveMongoTemplate tempate) {
+		this.tempate = tempate;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -54,14 +53,20 @@ class ReactiveRemoveOperationSupport implements ReactiveRemoveOperation {
 		return new ReactiveRemoveSupport<>(tempate, domainType, ALL_QUERY, null);
 	}
 
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ReactiveRemoveSupport<T> implements ReactiveRemove<T>, RemoveWithCollection<T> {
 
-		@NonNull ReactiveMongoTemplate template;
-		@NonNull Class<T> domainType;
-		Query query;
-		String collection;
+		private final ReactiveMongoTemplate template;
+		private final Class<T> domainType;
+		private final Query query;
+		private final String collection;
+
+		ReactiveRemoveSupport(ReactiveMongoTemplate template, Class<T> domainType, Query query, String collection) {
+
+			this.template = template;
+			this.domainType = domainType;
+			this.query = query;
+			this.collection = collection;
+		}
 
 		/*
 		 * (non-Javadoc)

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

@@ -15,13 +15,10 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-import lombok.experimental.FieldDefaults;
 import reactor.core.publisher.Mono;
 
 import org.springframework.data.mongodb.core.query.Query;
+import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.StringUtils;
@@ -35,12 +32,15 @@ import com.mongodb.client.result.UpdateResult;
  * @author Christoph Strobl
  * @since 2.0
  */
-@RequiredArgsConstructor
 class ReactiveUpdateOperationSupport implements ReactiveUpdateOperation {
 
 	private static final Query ALL_QUERY = new Query();
 
-	private final @NonNull ReactiveMongoTemplate template;
+	private final ReactiveMongoTemplate template;
+
+	ReactiveUpdateOperationSupport(ReactiveMongoTemplate template) {
+		this.template = template;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -54,21 +54,34 @@ class ReactiveUpdateOperationSupport implements ReactiveUpdateOperation {
 		return new ReactiveUpdateSupport<>(template, domainType, ALL_QUERY, null, null, null, null, null, domainType);
 	}
 
-	@RequiredArgsConstructor
-	@FieldDefaults(level = AccessLevel.PRIVATE, makeFinal = true)
 	static class ReactiveUpdateSupport<T>
 			implements ReactiveUpdate<T>, UpdateWithCollection<T>, UpdateWithQuery<T>, TerminatingUpdate<T>,
 			FindAndReplaceWithOptions<T>, FindAndReplaceWithProjection<T>, TerminatingFindAndReplace<T> {
 
-		@NonNull ReactiveMongoTemplate template;
-		@NonNull Class<?> domainType;
-		Query query;
-		org.springframework.data.mongodb.core.query.UpdateDefinition update;
-		@Nullable String collection;
-		@Nullable FindAndModifyOptions findAndModifyOptions;
-		@Nullable FindAndReplaceOptions findAndReplaceOptions;
-		@Nullable Object replacement;
-		@NonNull Class<T> targetType;
+		private final ReactiveMongoTemplate template;
+		private final Class<?> domainType;
+		private final Query query;
+		private final org.springframework.data.mongodb.core.query.UpdateDefinition update;
+		@Nullable private final String collection;
+		@Nullable private final FindAndModifyOptions findAndModifyOptions;
+		@Nullable private final FindAndReplaceOptions findAndReplaceOptions;
+		@Nullable private final Object replacement;
+		private final Class<T> targetType;
+
+		ReactiveUpdateSupport(ReactiveMongoTemplate template, Class<?> domainType, Query query, UpdateDefinition update,
+				String collection, FindAndModifyOptions findAndModifyOptions, FindAndReplaceOptions findAndReplaceOptions,
+				Object replacement, Class<T> targetType) {
+
+			this.template = template;
+			this.domainType = domainType;
+			this.query = query;
+			this.update = update;
+			this.collection = collection;
+			this.findAndModifyOptions = findAndModifyOptions;
+			this.findAndReplaceOptions = findAndReplaceOptions;
+			this.replacement = replacement;
+			this.targetType = targetType;
+		}
 
 		/*
 		 * (non-Javadoc)
@@ -123,7 +136,9 @@ class ReactiveUpdateOperationSupport implements ReactiveUpdateOperation {
 
 			String collectionName = getCollectionName();
 
-			return template.findAndModify(query, update, findAndModifyOptions != null ? findAndModifyOptions : FindAndModifyOptions.none(), targetType, collectionName);
+			return template.findAndModify(query, update,
+					findAndModifyOptions != null ? findAndModifyOptions : FindAndModifyOptions.none(), targetType,
+					collectionName);
 		}
 
 		/*

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

@@ -15,7 +15,6 @@
  */
 package org.springframework.data.mongodb.core;
 
-import lombok.Value;
 import reactor.core.publisher.Mono;
 
 import org.bson.codecs.configuration.CodecRegistry;
@@ -27,6 +26,7 @@ import org.springframework.data.mongodb.ReactiveMongoDatabaseFactory;
 import org.springframework.data.mongodb.SessionAwareMethodInterceptor;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 
 import com.mongodb.ClientSessionOptions;
 import com.mongodb.ConnectionString;
@@ -175,11 +175,16 @@ public class SimpleReactiveMongoDatabaseFactory implements DisposableBean, React
 	 * @author Christoph Strobl
 	 * @since 2.1
 	 */
-	@Value
-	static class ClientSessionBoundMongoDbFactory implements ReactiveMongoDatabaseFactory {
+	static final class ClientSessionBoundMongoDbFactory implements ReactiveMongoDatabaseFactory {
 
-		ClientSession session;
-		ReactiveMongoDatabaseFactory delegate;
+		private final ClientSession session;
+		private final ReactiveMongoDatabaseFactory delegate;
+
+		ClientSessionBoundMongoDbFactory(ClientSession session, ReactiveMongoDatabaseFactory delegate) {
+
+			this.session = session;
+			this.delegate = delegate;
+		}
 
 		/*
 		 * (non-Javadoc)
@@ -266,7 +271,42 @@ 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()));
+		}
+
+		public ClientSession getSession() {
+			return this.session;
+		}
+
+		public ReactiveMongoDatabaseFactory getDelegate() {
+			return this.delegate;
+		}
+
+		@Override
+		public boolean equals(Object o) {
+			if (this == o)
+				return true;
+			if (o == null || getClass() != o.getClass())
+				return false;
+
+			ClientSessionBoundMongoDbFactory that = (ClientSessionBoundMongoDbFactory) o;
+
+			if (!ObjectUtils.nullSafeEquals(this.session, that.session)) {
+				return false;
+			}
+			return ObjectUtils.nullSafeEquals(this.delegate, that.delegate);
+		}
+
+		@Override
+		public int hashCode() {
+			int result = ObjectUtils.nullSafeHashCode(this.session);
+			result = 31 * result + ObjectUtils.nullSafeHashCode(this.delegate);
+			return result;
+		}
+
+		public String toString() {
+			return "SimpleReactiveMongoDatabaseFactory.ClientSessionBoundMongoDbFactory(session=" + this.getSession()
+					+ ", delegate=" + this.getDelegate() + ")";
 		}
 	}
 }

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

@@ -29,8 +29,11 @@ import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
 /**
+ * Support class for {@link AggregationExpression} implementations.
+ *
  * @author Christoph Strobl
  * @author Matt Morrissette
+ * @author Mark Paluch
  * @since 1.10
  */
 abstract class AbstractAggregationExpression implements AggregationExpression {
@@ -49,7 +52,6 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 		return toDocument(this.value, context);
 	}
 
-	@SuppressWarnings("unchecked")
 	public Document toDocument(Object value, AggregationOperationContext context) {
 		return new Document(getMongoMethod(), unpack(value, context));
 	}
@@ -101,17 +103,19 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 		return value;
 	}
 
+	@SuppressWarnings("unchecked")
 	protected List<Object> append(Object value, Expand expandList) {
 
 		if (this.value instanceof List) {
 
-			List<Object> clone = new ArrayList<Object>((List) this.value);
+			List<Object> clone = new ArrayList<>((List<Object>) this.value);
 
 			if (value instanceof Collection && Expand.EXPAND_VALUES.equals(expandList)) {
 				clone.addAll((Collection<?>) value);
 			} else {
 				clone.add(value);
 			}
+
 			return clone;
 		}
 
@@ -129,25 +133,72 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 		return append(value, Expand.EXPAND_VALUES);
 	}
 
-	@SuppressWarnings("unchecked")
-	protected java.util.Map<String, Object> append(String key, Object value) {
+	@SuppressWarnings({ "unchecked", "rawtypes" })
+	protected Map<String, Object> append(String key, Object value) {
 
 		Assert.isInstanceOf(Map.class, this.value, "Value must be a type of Map!");
 
-		java.util.Map<String, Object> clone = new LinkedHashMap<>((java.util.Map) this.value);
+		Map<String, Object> clone = new LinkedHashMap<>((java.util.Map) this.value);
 		clone.put(key, value);
 		return clone;
 
 	}
 
+	@SuppressWarnings({ "unchecked", "rawtypes" })
+	protected Map<String, Object> remove(String key) {
+
+		Assert.isInstanceOf(Map.class, this.value, "Value must be a type of Map!");
+
+		Map<String, Object> clone = new LinkedHashMap<>((java.util.Map) this.value);
+		clone.remove(key);
+		return clone;
+	}
+
+	/**
+	 * Append the given key at the position in the underlying {@link LinkedHashMap}.
+	 *
+	 * @param index
+	 * @param key
+	 * @param value
+	 * @return
+	 * @since 3.1
+	 */
+	@SuppressWarnings({ "unchecked" })
+	protected Map<String, Object> appendAt(int index, String key, Object value) {
+
+		Assert.isInstanceOf(Map.class, this.value, "Value must be a type of Map!");
+
+		Map<String, Object> clone = new LinkedHashMap<>();
+
+		int i = 0;
+		for (Map.Entry<String, Object> entry : ((Map<String, Object>) this.value).entrySet()) {
+
+			if (i == index) {
+				clone.put(key, value);
+			}
+			if (!entry.getKey().equals(key)) {
+				clone.put(entry.getKey(), entry.getValue());
+			}
+			i++;
+		}
+		if (i <= index) {
+			clone.put(key, value);
+		}
+		return clone;
+
+	}
+
+	@SuppressWarnings({ "rawtypes" })
 	protected List<Object> values() {
 
 		if (value instanceof List) {
 			return new ArrayList<Object>((List) value);
 		}
+
 		if (value instanceof java.util.Map) {
 			return new ArrayList<Object>(((java.util.Map) value).values());
 		}
+
 		return new ArrayList<>(Collections.singletonList(value));
 	}
 
@@ -177,7 +228,7 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 
 		Assert.isInstanceOf(Map.class, this.value, "Value must be a type of Map!");
 
-		return (T) ((java.util.Map<String, Object>) this.value).get(key);
+		return (T) ((Map<String, Object>) this.value).get(key);
 	}
 
 	/**
@@ -187,11 +238,11 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 	 * @return
 	 */
 	@SuppressWarnings("unchecked")
-	protected java.util.Map<String, Object> argumentMap() {
+	protected Map<String, Object> argumentMap() {
 
 		Assert.isInstanceOf(Map.class, this.value, "Value must be a type of Map!");
 
-		return Collections.unmodifiableMap((java.util.Map) value);
+		return Collections.unmodifiableMap((java.util.Map<String, Object>) value);
 	}
 
 	/**
@@ -208,7 +259,7 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 			return false;
 		}
 
-		return ((java.util.Map<String, Object>) this.value).containsKey(key);
+		return ((Map<String, Object>) this.value).containsKey(key);
 	}
 
 	protected abstract String getMongoMethod();

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;
 
 	/**
@@ -139,7 +139,7 @@ public class Aggregation {
 	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}.
 	 *
@@ -661,9 +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));
@@ -718,7 +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;
 	}
 
 	/**

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/AggregationOptions.java

@@ -32,6 +32,7 @@ import org.springframework.util.Assert;
  * @author Oliver Gierke
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Yadhukrishna S Pai
  * @see Aggregation#withOptions(AggregationOptions)
  * @see TypedAggregation#withOptions(AggregationOptions)
  * @since 1.6
@@ -45,13 +46,16 @@ public class AggregationOptions {
 	private static final String COLLATION = "collation";
 	private static final String COMMENT = "comment";
 	private static final String MAX_TIME = "maxTimeMS";
+	private static final String HINT = "hint";
 
 	private final boolean allowDiskUse;
 	private final boolean explain;
 	private final Optional<Document> cursor;
 	private final Optional<Collation> collation;
 	private final Optional<String> comment;
+	private final Optional<Document> hint;
 	private Duration maxTime = Duration.ZERO;
+	private ResultOptions resultOptions = ResultOptions.READ;
 
 	/**
 	 * Creates a new {@link AggregationOptions}.
@@ -70,13 +74,13 @@ 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
 	 */
 	public AggregationOptions(boolean allowDiskUse, boolean explain, @Nullable Document cursor,
 			@Nullable Collation collation) {
-		this(allowDiskUse, explain, cursor, collation, null);
+		this(allowDiskUse, explain, cursor, collation, null, null);
 	}
 
 	/**
@@ -85,19 +89,37 @@ 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
 	 */
 	public AggregationOptions(boolean allowDiskUse, boolean explain, @Nullable Document cursor,
 			@Nullable Collation collation, @Nullable String comment) {
+		this(allowDiskUse, explain, cursor, collation, comment, null);
+	}
+
+	/**
+	 * Creates a new {@link 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.
+	 * @param collation collation for string comparison. Can be {@literal null}.
+	 * @param comment execution comment. Can be {@literal null}.
+	 * @param hint can be {@literal null}, used to provide an index that would be forcibly used by query optimizer.
+	 * @since 3.1
+	 */
+	private AggregationOptions(boolean allowDiskUse, boolean explain, @Nullable Document cursor,
+			@Nullable Collation collation, @Nullable String comment, @Nullable Document hint) {
 
 		this.allowDiskUse = allowDiskUse;
 		this.explain = explain;
 		this.cursor = Optional.ofNullable(cursor);
 		this.collation = Optional.ofNullable(collation);
 		this.comment = Optional.ofNullable(comment);
+		this.hint = Optional.ofNullable(hint);
 	}
 
 	/**
@@ -129,8 +151,9 @@ public class AggregationOptions {
 		Collation collation = document.containsKey(COLLATION) ? Collation.from(document.get(COLLATION, Document.class))
 				: null;
 		String comment = document.getString(COMMENT);
+		Document hint = document.get(HINT, Document.class);
 
-		AggregationOptions options = new AggregationOptions(allowDiskUse, explain, cursor, collation, comment);
+		AggregationOptions options = new AggregationOptions(allowDiskUse, explain, cursor, collation, comment, hint);
 		if (document.containsKey(MAX_TIME)) {
 			options.maxTime = Duration.ofMillis(document.getLong(MAX_TIME));
 		}
@@ -211,6 +234,16 @@ public class AggregationOptions {
 		return comment;
 	}
 
+	/**
+	 * Get the hint used to to fulfill the aggregation.
+	 *
+	 * @return never {@literal null}.
+	 * @since 3.1
+	 */
+	public Optional<Document> getHint() {
+		return hint;
+	}
+
 	/**
 	 * @return the time limit for processing. {@link Duration#ZERO} is used for the default unbounded behavior.
 	 * @since 3.0
@@ -219,6 +252,15 @@ 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.
@@ -238,6 +280,10 @@ public class AggregationOptions {
 			result.put(EXPLAIN, explain);
 		}
 
+		if (result.containsKey(HINT)) {
+			hint.ifPresent(val -> result.append(HINT, val));
+		}
+
 		if (!result.containsKey(CURSOR)) {
 			cursor.ifPresent(val -> result.put(CURSOR, val));
 		}
@@ -267,6 +313,7 @@ public class AggregationOptions {
 		cursor.ifPresent(val -> document.put(CURSOR, val));
 		collation.ifPresent(val -> document.append(COLLATION, val.toDocument()));
 		comment.ifPresent(val -> document.append(COMMENT, val));
+		hint.ifPresent(val -> document.append(HINT, val));
 
 		if (hasExecutionTimeLimit()) {
 			document.append(MAX_TIME, maxTime.toMillis());
@@ -308,7 +355,9 @@ public class AggregationOptions {
 		private @Nullable Document cursor;
 		private @Nullable Collation collation;
 		private @Nullable String comment;
+		private @Nullable Document hint;
 		private @Nullable Duration maxTime;
+		private @Nullable ResultOptions resultOptions;
 
 		/**
 		 * Defines whether to off-load intensive sort-operations to disk.
@@ -385,11 +434,24 @@ public class AggregationOptions {
 			return this;
 		}
 
+		/**
+		 * Define a hint that is used by query optimizer to to fulfill the aggregation.
+		 *
+		 * @param hint can be {@literal null}.
+		 * @return this.
+		 * @since 3.1
+		 */
+		public Builder hint(@Nullable Document hint) {
+
+			this.hint = hint;
+			return this;
+		}
+
 		/**
 		 * 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
 		 */
@@ -399,6 +461,20 @@ 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.
 		 *
@@ -406,12 +482,30 @@ public class AggregationOptions {
 		 */
 		public AggregationOptions build() {
 
-			AggregationOptions options = new AggregationOptions(allowDiskUse, explain, cursor, collation, comment);
+			AggregationOptions options = new AggregationOptions(allowDiskUse, explain, cursor, collation, comment, hint);
 			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 +1,162 @@
+/*
+ * 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 java.util.function.Predicate;
+
+import org.bson.Document;
+import org.springframework.util.Assert;
+
+/**
+ * The {@link AggregationPipeline} holds the collection of {@link AggregationOperation aggregation stages}.
+ *
+ * @author Christoph Strobl
+ * @author Mark Paluch
+ * @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) {
+
+		Assert.notNull(aggregationOperations, "AggregationOperations must not be null!");
+		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 (isEmpty()) {
+			return false;
+		}
+
+		AggregationOperation operation = pipeline.get(pipeline.size() - 1);
+		return isOut(operation) || isMerge(operation);
+	}
+
+	void verify() {
+
+		// check $out/$merge is the last operation if it exists
+		for (AggregationOperation operation : pipeline) {
+
+			if (isOut(operation) && !isLast(operation)) {
+				throw new IllegalArgumentException("The $out operator must be the last stage in the pipeline.");
+			}
+
+			if (isMerge(operation) && !isLast(operation)) {
+				throw new IllegalArgumentException("The $merge operator must be the last stage in the pipeline.");
+			}
+		}
+	}
+
+	/**
+	 * Return whether this aggregation pipeline defines a {@code $unionWith} stage that may contribute documents from
+	 * other collections. Checking for presence of union stages is useful when attempting to determine the aggregation
+	 * element type for mapping metadata computation.
+	 *
+	 * @return {@literal true} the aggregation pipeline makes use of {@code $unionWith}.
+	 * @since 3.1
+	 */
+	public boolean containsUnionWith() {
+		return containsOperation(AggregationPipeline::isUnionWith);
+	}
+
+	/**
+	 * @return {@literal true} if the pipeline does not contain any stages.
+	 * @since 3.1
+	 */
+	public boolean isEmpty() {
+		return pipeline.isEmpty();
+	}
+
+	private boolean containsOperation(Predicate<AggregationOperation> predicate) {
+
+		if (isEmpty()) {
+			return false;
+		}
+
+		for (AggregationOperation element : pipeline) {
+			if (predicate.test(element)) {
+				return true;
+			}
+		}
+
+		return false;
+	}
+
+	private boolean isLast(AggregationOperation aggregationOperation) {
+		return pipeline.indexOf(aggregationOperation) == pipeline.size() - 1;
+	}
+
+	private static boolean isUnionWith(AggregationOperation operator) {
+		return operator instanceof UnionWithOperation || operator.getOperator().equals("$unionWith");
+	}
+
+	private static boolean isMerge(AggregationOperation operator) {
+		return operator instanceof MergeOperation || operator.getOperator().equals("$merge");
+	}
+
+	private static boolean isOut(AggregationOperation operator) {
+		return operator instanceof OutOperation || operator.getOperator().equals("$out");
+	}
+}

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;
 	}
@@ -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;
 	}
 

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
@@ -106,7 +105,16 @@ 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";
 	}
 
 	/**

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
@@ -103,7 +101,16 @@ 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";
 	}
 
 	/**
@@ -204,8 +211,8 @@ 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}.

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

@@ -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)

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/ExposedFieldsAggregationOperationContext.java

@@ -149,4 +149,12 @@ class ExposedFieldsAggregationOperationContext implements AggregationOperationCo
 		}
 		return null;
 	}
+
+	/**
+	 * @return obtain the root context used to resolve references.
+	 * @since 3.1
+	 */
+	AggregationOperationContext getRootContext() {
+		return rootContext;
+	}
 }

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";
 	}
 
 	/*

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

@@ -83,7 +83,16 @@ 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";
 	}
 
 	/**
@@ -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";
 	}
 
 	/*

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;

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/ScriptOperators.java

@@ -0,0 +1,587 @@
+/*
+ * 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.Arrays;
+import java.util.Collections;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
+import org.springframework.data.mongodb.core.aggregation.ScriptOperators.Accumulator.AccumulatorBuilder;
+import org.springframework.data.mongodb.core.aggregation.ScriptOperators.Accumulator.AccumulatorInitBuilder;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+import org.springframework.util.CollectionUtils;
+
+/**
+ * Gateway to {@literal $function} and {@literal $accumulator} aggregation operations.
+ * <p />
+ * Using {@link ScriptOperators} as part of the {@link Aggregation} requires MongoDB server to have
+ * <a href="https://docs.mongodb.com/master/core/server-side-javascript/">server-side JavaScript</a> execution
+ * <a href="https://docs.mongodb.com/master/reference/configuration-options/#security.javascriptEnabled">enabled</a>.
+ *
+ * @author Christoph Strobl
+ * @author Mark Paluch
+ * @since 3.1
+ */
+public class ScriptOperators {
+
+	/**
+	 * Create a custom aggregation
+	 * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/function/">$function<a /> in JavaScript.
+	 *
+	 * @param body The function definition. Must not be {@literal null}.
+	 * @return new instance of {@link Function}.
+	 */
+	public static Function function(String body) {
+		return Function.function(body);
+	}
+
+	/**
+	 * Create a custom <a href="https://docs.mongodb.com/master/reference/operator/aggregation/accumulator/">$accumulator
+	 * operator</a> in Javascript.
+	 *
+	 * @return new instance of {@link AccumulatorInitBuilder}.
+	 */
+	public static AccumulatorInitBuilder accumulatorBuilder() {
+		return new AccumulatorBuilder();
+	}
+
+	/**
+	 * {@link Function} defines a custom aggregation
+	 * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/function/">$function</a> in JavaScript.
+	 * <p />
+	 * <code class="java">
+	 * {
+	 *   $function: {
+	 *     body: ...,
+	 *     args: ...,
+	 *     lang: "js"
+	 *   }
+	 * }
+	 * </code>
+	 * <p />
+	 * {@link Function} cannot be used as part of {@link org.springframework.data.mongodb.core.schema.MongoJsonSchema
+	 * schema} validation query expression. <br />
+	 * <b>NOTE:</b> <a href="https://docs.mongodb.com/master/core/server-side-javascript/">Server-Side JavaScript</a>
+	 * execution must be
+	 * <a href="https://docs.mongodb.com/master/reference/configuration-options/#security.javascriptEnabled">enabled</a>
+	 *
+	 * @see <a href="https://docs.mongodb.com/master/reference/operator/aggregation/function/">MongoDB Documentation:
+	 *      $function</a>
+	 */
+	public static class Function extends AbstractAggregationExpression {
+
+		private Function(Map<String, Object> values) {
+			super(values);
+		}
+
+		/**
+		 * Create a new {@link Function} with the given function definition.
+		 *
+		 * @param body must not be {@literal null}.
+		 * @return new instance of {@link Function}.
+		 */
+		public static Function function(String body) {
+
+			Assert.notNull(body, "Function body must not be null!");
+
+			Map<String, Object> function = new LinkedHashMap<>(2);
+			function.put(Fields.BODY.toString(), body);
+			function.put(Fields.ARGS.toString(), Collections.emptyList());
+			function.put(Fields.LANG.toString(), "js");
+
+			return new Function(function);
+		}
+
+		/**
+		 * Set the arguments passed to the function body.
+		 *
+		 * @param args the arguments passed to the function body. Leave empty if the function does not take any arguments.
+		 * @return new instance of {@link Function}.
+		 */
+		public Function args(Object... args) {
+			return args(Arrays.asList(args));
+		}
+
+		/**
+		 * Set the arguments passed to the function body.
+		 *
+		 * @param args the arguments passed to the function body. Leave empty if the function does not take any arguments.
+		 * @return new instance of {@link Function}.
+		 */
+		public Function args(List<Object> args) {
+
+			Assert.notNull(args, "Args must not be null! Use an empty list instead.");
+
+			return new Function(appendAt(1, Fields.ARGS.toString(), args));
+		}
+
+		/**
+		 * The language used in the body.
+		 *
+		 * @param lang must not be {@literal null} nor empty.
+		 * @return new instance of {@link Function}.
+		 */
+		public Function lang(String lang) {
+
+			Assert.hasText(lang, "Lang must not be null nor empty! The default would be 'js'.");
+
+			return new Function(appendAt(2, Fields.LANG.toString(), lang));
+		}
+
+		@Nullable
+		List<Object> getArgs() {
+			return get(Fields.ARGS.toString());
+		}
+
+		String getBody() {
+			return get(Fields.BODY.toString());
+		}
+
+		String getLang() {
+			return get(Fields.LANG.toString());
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$function";
+		}
+
+		enum Fields {
+
+			BODY, ARGS, LANG;
+
+			@Override
+			public String toString() {
+				return name().toLowerCase();
+			}
+		}
+	}
+
+	/**
+	 * {@link Accumulator} defines a custom aggregation
+	 * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/accumulator/">$accumulator operator</a>,
+	 * one that maintains its state (e.g. totals, maximums, minimums, and related data) as documents progress through the
+	 * pipeline, in JavaScript.
+	 * <p />
+	 * <code class="java">
+	 * {
+	 *   $accumulator: {
+	 *     init: ...,
+	 *     intArgs: ...,
+	 *     accumulate: ...,
+	 *     accumulateArgs: ...,
+	 *     merge: ...,
+	 *     finalize: ...,
+	 *     lang: "js"
+	 *   }
+	 * }
+	 * </code>
+	 * <p />
+	 * {@link Accumulator} can be used as part of {@link GroupOperation $group}, {@link BucketOperation $bucket} and
+	 * {@link BucketAutoOperation $bucketAuto} pipeline stages. <br />
+	 * <b>NOTE:</b> <a href="https://docs.mongodb.com/master/core/server-side-javascript/">Server-Side JavaScript</a>
+	 * execution must be
+	 * <a href="https://docs.mongodb.com/master/reference/configuration-options/#security.javascriptEnabled">enabled</a>
+	 *
+	 * @see <a href="https://docs.mongodb.com/master/reference/operator/aggregation/accumulator/">MongoDB Documentation:
+	 *      $accumulator</a>
+	 */
+	public static class Accumulator extends AbstractAggregationExpression {
+
+		private Accumulator(Map<String, Object> value) {
+			super(value);
+		}
+
+		@Override
+		protected String getMongoMethod() {
+			return "$accumulator";
+		}
+
+		enum Fields {
+
+			ACCUMULATE("accumulate"), //
+			ACCUMULATE_ARGS("accumulateArgs"), //
+			FINALIZE("finalize"), //
+			INIT("init"), //
+			INIT_ARGS("initArgs"), //
+			LANG("lang"), //
+			MERGE("merge"); //
+
+			private String field;
+
+			Fields(String field) {
+				this.field = field;
+			}
+
+			@Override
+			public String toString() {
+				return field;
+			}
+		}
+
+		public interface AccumulatorInitBuilder {
+
+			/**
+			 * Define the {@code init} {@link Function} for the {@link Accumulator accumulators} initial state. The function
+			 * receives its arguments from the {@link Function#args(Object...) initArgs} array expression.
+			 * <p />
+			 * <code class="java">
+			 * function(initArg1, initArg2, ...) {
+			 *   ...
+			 *   return initialState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			default AccumulatorAccumulateBuilder init(Function function) {
+				return init(function.getBody()).initArgs(function.getArgs());
+			}
+
+			/**
+			 * Define the {@code init} function for the {@link Accumulator accumulators} initial state. The function receives
+			 * its arguments from the {@link AccumulatorInitArgsBuilder#initArgs(Object...)} array expression.
+			 * <p />
+			 * <code class="java">
+			 * function(initArg1, initArg2, ...) {
+			 *   ...
+			 *   return initialState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			AccumulatorInitArgsBuilder init(String function);
+
+			/**
+			 * The language used in the {@code $accumulator} code.
+			 *
+			 * @param lang must not be {@literal null}. Default is {@literal js}.
+			 * @return this.
+			 */
+			AccumulatorInitBuilder lang(String lang);
+		}
+
+		public interface AccumulatorInitArgsBuilder extends AccumulatorAccumulateBuilder {
+
+			/**
+			 * Define the optional {@code initArgs} for the {@link AccumulatorInitBuilder#init(String)} function.
+			 *
+			 * @param args must not be {@literal null}.
+			 * @return this.
+			 */
+			default AccumulatorAccumulateBuilder initArgs(Object... args) {
+				return initArgs(Arrays.asList(args));
+			}
+
+			/**
+			 * Define the optional {@code initArgs} for the {@link AccumulatorInitBuilder#init(String)} function.
+			 *
+			 * @param args must not be {@literal null}.
+			 * @return this.
+			 */
+			AccumulatorAccumulateBuilder initArgs(List<Object> args);
+		}
+
+		public interface AccumulatorAccumulateBuilder {
+
+			/**
+			 * Set the {@code accumulate} {@link Function} that updates the state for each document. The functions first
+			 * argument is the current {@code state}, additional arguments can be defined via {@link Function#args(Object...)
+			 * accumulateArgs}.
+			 * <p />
+			 * <code class="java">
+			 * function(state, accumArg1, accumArg2, ...) {
+			 *   ...
+			 *   return newState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			default AccumulatorMergeBuilder accumulate(Function function) {
+				return accumulate(function.getBody()).accumulateArgs(function.getArgs());
+			}
+
+			/**
+			 * Set the {@code accumulate} function that updates the state for each document. The functions first argument is
+			 * the current {@code state}, additional arguments can be defined via
+			 * {@link AccumulatorAccumulateArgsBuilder#accumulateArgs(Object...)}.
+			 * <p />
+			 * <code class="java">
+			 * function(state, accumArg1, accumArg2, ...) {
+			 *   ...
+			 *   return newState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			AccumulatorAccumulateArgsBuilder accumulate(String function);
+		}
+
+		public interface AccumulatorAccumulateArgsBuilder extends AccumulatorMergeBuilder {
+
+			/**
+			 * Define additional {@code accumulateArgs} for the {@link AccumulatorAccumulateBuilder#accumulate(String)}
+			 * function.
+			 *
+			 * @param args must not be {@literal null}.
+			 * @return this.
+			 */
+			default AccumulatorMergeBuilder accumulateArgs(Object... args) {
+				return accumulateArgs(Arrays.asList(args));
+			}
+
+			/**
+			 * Define additional {@code accumulateArgs} for the {@link AccumulatorAccumulateBuilder#accumulate(String)}
+			 * function.
+			 *
+			 * @param args must not be {@literal null}.
+			 * @return this.
+			 */
+			AccumulatorMergeBuilder accumulateArgs(List<Object> args);
+		}
+
+		public interface AccumulatorMergeBuilder {
+
+			/**
+			 * Set the {@code merge} function used to merge two internal states. <br />
+			 * This might be required because the operation is run on a sharded cluster or when the operator exceeds its
+			 * memory limit.
+			 * <p />
+			 * <code class="java">
+			 * function(state1, state2) {
+			 *   ...
+			 *   return newState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			AccumulatorFinalizeBuilder merge(String function);
+		}
+
+		public interface AccumulatorFinalizeBuilder {
+
+			/**
+			 * Set the {@code finalize} function used to update the result of the accumulation when all documents have been
+			 * processed.
+			 * <p />
+			 * <code class="java">
+			 * function(state) {
+			 *   ...
+			 *   return finalState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return new instance of {@link Accumulator}.
+			 */
+			Accumulator finalize(String function);
+
+			/**
+			 * Build the {@link Accumulator} object without specifying a {@link #finalize(String) finalize function}.
+			 *
+			 * @return new instance of {@link Accumulator}.
+			 */
+			Accumulator build();
+		}
+
+		static class AccumulatorBuilder
+				implements AccumulatorInitBuilder, AccumulatorInitArgsBuilder, AccumulatorAccumulateBuilder,
+				AccumulatorAccumulateArgsBuilder, AccumulatorMergeBuilder, AccumulatorFinalizeBuilder {
+
+			private List<Object> initArgs;
+			private String initFunction;
+			private List<Object> accumulateArgs;
+			private String accumulateFunction;
+			private String mergeFunction;
+			private String finalizeFunction;
+			private String lang = "js";
+
+			/**
+			 * Define the {@code init} function for the {@link Accumulator accumulators} initial state. The function receives
+			 * its arguments from the {@link #initArgs(Object...)} array expression.
+			 * <p />
+			 * <code class="java">
+			 * function(initArg1, initArg2, ...) {
+			 *   ...
+			 *   return initialState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			@Override
+			public AccumulatorBuilder init(String function) {
+
+				this.initFunction = function;
+				return this;
+			}
+
+			/**
+			 * Define the optional {@code initArgs} for the {@link #init(String)} function.
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			@Override
+			public AccumulatorBuilder initArgs(List<Object> args) {
+
+				Assert.notNull(args, "Args must not be null");
+
+				this.initArgs = new ArrayList<>(args);
+				return this;
+			}
+
+			/**
+			 * Set the {@code accumulate} function that updates the state for each document. The functions first argument is
+			 * the current {@code state}, additional arguments can be defined via {@link #accumulateArgs(Object...)}.
+			 * <p />
+			 * <code class="java">
+			 * function(state, accumArg1, accumArg2, ...) {
+			 *   ...
+			 *   return newState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			@Override
+			public AccumulatorBuilder accumulate(String function) {
+
+				Assert.notNull(function, "Accumulate function must not be null");
+
+				this.accumulateFunction = function;
+				return this;
+			}
+
+			/**
+			 * Define additional {@code accumulateArgs} for the {@link #accumulate(String)} function.
+			 *
+			 * @param args must not be {@literal null}.
+			 * @return this.
+			 */
+			@Override
+			public AccumulatorBuilder accumulateArgs(List<Object> args) {
+
+				Assert.notNull(args, "Args must not be null");
+
+				this.accumulateArgs = new ArrayList<>(args);
+				return this;
+			}
+
+			/**
+			 * Set the {@code merge} function used to merge two internal states. <br />
+			 * This might be required because the operation is run on a sharded cluster or when the operator exceeds its
+			 * memory limit.
+			 * <p />
+			 * <code class="java">
+			 * function(state1, state2) {
+			 *   ...
+			 *   return newState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return this.
+			 */
+			@Override
+			public AccumulatorBuilder merge(String function) {
+
+				Assert.notNull(function, "Merge function must not be null");
+
+				this.mergeFunction = function;
+				return this;
+			}
+
+			/**
+			 * The language used in the {@code $accumulator} code.
+			 *
+			 * @param lang must not be {@literal null}. Default is {@literal js}.
+			 * @return this.
+			 */
+			public AccumulatorBuilder lang(String lang) {
+
+				Assert.hasText(lang, "Lang must not be null nor empty! The default would be 'js'.");
+
+				this.lang = lang;
+				return this;
+			}
+
+			/**
+			 * Set the {@code finalize} function used to update the result of the accumulation when all documents have been
+			 * processed.
+			 * <p />
+			 * <code class="java">
+			 * function(state) {
+			 *   ...
+			 *   return finalState
+			 * }
+			 * </code>
+			 *
+			 * @param function must not be {@literal null}.
+			 * @return new instance of {@link Accumulator}.
+			 */
+			@Override
+			public Accumulator finalize(String function) {
+
+				Assert.notNull(function, "Finalize function must not be null");
+
+				this.finalizeFunction = function;
+
+				Map<String, Object> args = createArgumentMap();
+				args.put(Fields.FINALIZE.toString(), finalizeFunction);
+
+				return new Accumulator(args);
+			}
+
+			@Override
+			public Accumulator build() {
+				return new Accumulator(createArgumentMap());
+			}
+
+			private Map<String, Object> createArgumentMap() {
+
+				Map<String, Object> args = new LinkedHashMap<>();
+				args.put(Fields.INIT.toString(), initFunction);
+				if (!CollectionUtils.isEmpty(initArgs)) {
+					args.put(Fields.INIT_ARGS.toString(), initArgs);
+				}
+				args.put(Fields.ACCUMULATE.toString(), accumulateFunction);
+				if (!CollectionUtils.isEmpty(accumulateArgs)) {
+					args.put(Fields.ACCUMULATE_ARGS.toString(), accumulateArgs);
+				}
+				args.put(Fields.MERGE.toString(), mergeFunction);
+				args.put(Fields.LANG.toString(), lang);
+
+				return args;
+			}
+		}
+	}
+}

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/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/TypeBasedAggregationOperationContext.java

@@ -133,6 +133,19 @@ public class TypeBasedAggregationOperationContext implements AggregationOperatio
 	 */
 	@Override
 	public AggregationOperationContext continueOnMissingFieldReference() {
+		return continueOnMissingFieldReference(type);
+	}
+
+	/**
+	 * This toggle allows the {@link AggregationOperationContext context} to use any given field name without checking for
+	 * its existence. Typically the {@link AggregationOperationContext} fails when referencing unknown fields, those that
+	 * are not present in one of the previous stages or the input source, throughout the pipeline.
+	 *
+	 * @param type The domain type to map fields to.
+	 * @return a more relaxed {@link AggregationOperationContext}.
+	 * @since 3.1
+	 */
+	public AggregationOperationContext continueOnMissingFieldReference(Class<?> type) {
 		return new RelaxedTypeBasedAggregationOperationContext(type, mappingContext, mapper);
 	}
 

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/UnionWithOperation.java

@@ -0,0 +1,168 @@
+/*
+ * 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.Arrays;
+import java.util.List;
+
+import org.bson.Document;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+
+/**
+ * The <a href="https://docs.mongodb.com/master/reference/operator/aggregation/unionWith/">$unionWith</a> aggregation
+ * stage (available since MongoDB 4.4) performs a union of two collections by combining pipeline results, potentially
+ * containing duplicates, into a single result set that is handed over to the next stage. <br />
+ * In order to remove duplicates it is possible to append a {@link GroupOperation} right after
+ * {@link UnionWithOperation}.
+ * <p />
+ * If the {@link UnionWithOperation} uses a
+ * <a href="https://docs.mongodb.com/master/reference/operator/aggregation/unionWith/#unionwith-pipeline">pipeline</a>
+ * to process documents, field names within the pipeline will be treated as is. In order to map domain type property
+ * names to actual field names (considering potential {@link org.springframework.data.mongodb.core.mapping.Field}
+ * annotations) make sure the enclosing aggregation is a {@link TypedAggregation} and provide the target type for the
+ * {@code $unionWith} stage via {@link #mapFieldsTo(Class)}.
+ *
+ * @author Christoph Strobl
+ * @see <a href="https://docs.mongodb.com/master/reference/operator/aggregation/unionWith/">Aggregation Pipeline Stage:
+ *      $unionWith</a>
+ * @since 3.1
+ */
+public class UnionWithOperation implements AggregationOperation {
+
+	private final String collection;
+
+	private final @Nullable AggregationPipeline pipeline;
+
+	private final @Nullable Class<?> domainType;
+
+	public UnionWithOperation(String collection, @Nullable AggregationPipeline pipeline, @Nullable Class<?> domainType) {
+
+		Assert.notNull(collection, "Collection must not be null!");
+
+		this.collection = collection;
+		this.pipeline = pipeline;
+		this.domainType = domainType;
+	}
+
+	/**
+	 * Set the name of the collection from which pipeline results should be included in the result set.<br />
+	 * The collection name is used to set the {@code coll} parameter of {@code $unionWith}.
+	 *
+	 * @param collection the MongoDB collection name. Must not be {@literal null}.
+	 * @return new instance of {@link UnionWithOperation}.
+	 * @throws IllegalArgumentException if the required argument is {@literal null}.
+	 */
+	public static UnionWithOperation unionWith(String collection) {
+		return new UnionWithOperation(collection, null, null);
+	}
+
+	/**
+	 * Set the {@link AggregationPipeline} to apply to the specified collection. The pipeline corresponds to the optional
+	 * {@code pipeline} field of the {@code $unionWith} aggregation stage and is used to compute the documents going into
+	 * the result set.
+	 *
+	 * @param pipeline the {@link AggregationPipeline} that computes the documents. Must not be {@literal null}.
+	 * @return new instance of {@link UnionWithOperation}.
+	 * @throws IllegalArgumentException if the required argument is {@literal null}.
+	 */
+	public UnionWithOperation pipeline(AggregationPipeline pipeline) {
+		return new UnionWithOperation(collection, pipeline, domainType);
+	}
+
+	/**
+	 * Set the aggregation pipeline stages to apply to the specified collection. The pipeline corresponds to the optional
+	 * {@code pipeline} field of the {@code $unionWith} aggregation stage and is used to compute the documents going into
+	 * the result set.
+	 *
+	 * @param aggregationStages the aggregation pipeline stages that compute the documents. Must not be {@literal null}.
+	 * @return new instance of {@link UnionWithOperation}.
+	 * @throws IllegalArgumentException if the required argument is {@literal null}.
+	 */
+	public UnionWithOperation pipeline(List<AggregationOperation> aggregationStages) {
+		return new UnionWithOperation(collection, new AggregationPipeline(aggregationStages), domainType);
+	}
+
+	/**
+	 * Set the aggregation pipeline stages to apply to the specified collection. The pipeline corresponds to the optional
+	 * {@code pipeline} field of the {@code $unionWith} aggregation stage and is used to compute the documents going into
+	 * the result set.
+	 *
+	 * @param aggregationStages the aggregation pipeline stages that compute the documents. Must not be {@literal null}.
+	 * @return new instance of {@link UnionWithOperation}.
+	 * @throws IllegalArgumentException if the required argument is {@literal null}.
+	 */
+	public UnionWithOperation pipeline(AggregationOperation... aggregationStages) {
+		return new UnionWithOperation(collection, new AggregationPipeline(Arrays.asList(aggregationStages)), domainType);
+	}
+
+	/**
+	 * Set domain type used for field name mapping of property references used by the {@link AggregationPipeline}.
+	 * Remember to also use a {@link TypedAggregation} in the outer pipeline.<br />
+	 * If not set, field names used within {@link AggregationOperation pipeline operations} are taken as is.
+	 *
+	 * @param domainType the domain type to map field names used in pipeline operations to. Must not be {@literal null}.
+	 * @return new instance of {@link UnionWithOperation}.
+	 * @throws IllegalArgumentException if the required argument is {@literal null}.
+	 */
+	public UnionWithOperation mapFieldsTo(Class<?> domainType) {
+
+		Assert.notNull(domainType, "DomainType must not be null!");
+		return new UnionWithOperation(collection, pipeline, domainType);
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#toDocument(org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+	 */
+	@Override
+	public Document toDocument(AggregationOperationContext context) {
+
+		Document $unionWith = new Document("coll", collection);
+		if (pipeline == null || pipeline.isEmpty()) {
+			return new Document(getOperator(), $unionWith);
+		}
+
+		$unionWith.append("pipeline", pipeline.toDocuments(computeContext(context)));
+		return new Document(getOperator(), $unionWith);
+	}
+
+	private AggregationOperationContext computeContext(AggregationOperationContext source) {
+
+		if (domainType == null) {
+			return Aggregation.DEFAULT_CONTEXT;
+		}
+
+		if (source instanceof TypeBasedAggregationOperationContext) {
+			return ((TypeBasedAggregationOperationContext) source).continueOnMissingFieldReference(domainType);
+		}
+
+		if (source instanceof ExposedFieldsAggregationOperationContext) {
+			return computeContext(((ExposedFieldsAggregationOperationContext) source).getRootContext());
+		}
+
+		return source;
+	}
+
+	/*
+	 * (non-Javadoc)
+	 * @see org.springframework.data.mongodb.core.aggregation.AggregationOperation#getOperator()
+	 */
+	@Override
+	public String getOperator() {
+		return "$unionWith";
+	}
+}

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/DefaultDbRefResolver.java

@@ -45,6 +45,7 @@ import org.springframework.dao.support.PersistenceExceptionTranslator;
 import org.springframework.data.mongodb.ClientSessionException;
 import org.springframework.data.mongodb.LazyLoadingException;
 import org.springframework.data.mongodb.MongoDatabaseFactory;
+import org.springframework.data.mongodb.MongoDatabaseUtils;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.lang.Nullable;
 import org.springframework.objenesis.ObjenesisStd;
@@ -114,14 +115,16 @@ public class DefaultDbRefResolver implements DbRefResolver {
 	@Override
 	public Document fetch(DBRef dbRef) {
 
+		MongoCollection<Document> mongoCollection = getCollection(dbRef);
+
 		if (LOGGER.isTraceEnabled()) {
 			LOGGER.trace("Fetching DBRef '{}' from {}.{}.", dbRef.getId(),
-					StringUtils.hasText(dbRef.getDatabaseName()) ? dbRef.getDatabaseName() : mongoDbFactory.getMongoDatabase().getName(),
+					StringUtils.hasText(dbRef.getDatabaseName()) ? dbRef.getDatabaseName()
+							: mongoCollection.getNamespace().getDatabaseName(),
 					dbRef.getCollectionName());
 		}
 
-		StringUtils.hasText(dbRef.getDatabaseName());
-		return getCollection(dbRef).find(Filters.eq("_id", dbRef.getId())).first();
+		return mongoCollection.find(Filters.eq("_id", dbRef.getId())).first();
 	}
 
 	/*
@@ -152,15 +155,16 @@ public class DefaultDbRefResolver implements DbRefResolver {
 		}
 
 		DBRef databaseSource = refs.iterator().next();
+		MongoCollection<Document> mongoCollection = getCollection(databaseSource);
 
 		if (LOGGER.isTraceEnabled()) {
 			LOGGER.trace("Bulk fetching DBRefs {} from {}.{}.", ids,
 					StringUtils.hasText(databaseSource.getDatabaseName()) ? databaseSource.getDatabaseName()
-							: mongoDbFactory.getMongoDatabase().getName(),
+							: mongoCollection.getNamespace().getDatabaseName(),
 					databaseSource.getCollectionName());
 		}
 
-		List<Document> result = getCollection(databaseSource) //
+		List<Document> result = mongoCollection //
 				.find(new Document("_id", new Document("$in", ids))) //
 				.into(new ArrayList<>());
 
@@ -202,7 +206,7 @@ public class DefaultDbRefResolver implements DbRefResolver {
 		proxyFactory.addInterface(propertyType);
 		proxyFactory.addAdvice(interceptor);
 
-		return handler.populateId(property, dbref, proxyFactory.getProxy());
+		return handler.populateId(property, dbref, proxyFactory.getProxy(LazyLoadingProxy.class.getClassLoader()));
 	}
 
 	/**
@@ -497,7 +501,7 @@ public class DefaultDbRefResolver implements DbRefResolver {
 	 */
 	protected MongoCollection<Document> getCollection(DBRef dbref) {
 
-		return (StringUtils.hasText(dbref.getDatabaseName()) ? mongoDbFactory.getMongoDatabase(dbref.getDatabaseName())
-				: mongoDbFactory.getMongoDatabase()).getCollection(dbref.getCollectionName(), Document.class);
+		return MongoDatabaseUtils.getDatabase(dbref.getDatabaseName(), mongoDbFactory)
+				.getCollection(dbref.getCollectionName(), Document.class);
 	}
 }

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

@@ -127,8 +127,8 @@ public interface MongoConverter
 	@Nullable
 	default Object convertId(@Nullable Object id, Class<?> targetType) {
 
-		if (id == null) {
-			return null;
+		if (id == null || ClassUtils.isAssignableValue(targetType, id)) {
+			return id;
 		}
 
 		if (ClassUtils.isAssignable(ObjectId.class, targetType)) {

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

@@ -965,7 +965,7 @@ public class QueryMapper {
 			this.entity = entity;
 			this.mappingContext = context;
 
-			this.path = getPath(removePlaceholders(POSITIONAL_PARAMETER_PATTERN, name));
+			this.path = getPath(removePlaceholders(POSITIONAL_PARAMETER_PATTERN, name), property);
 			this.property = path == null ? property : path.getLeafProperty();
 			this.association = findAssociation();
 		}
@@ -1079,11 +1079,17 @@ public class QueryMapper {
 		 * @return
 		 */
 		@Nullable
-		private PersistentPropertyPath<MongoPersistentProperty> getPath(String pathExpression) {
+		private PersistentPropertyPath<MongoPersistentProperty> getPath(String pathExpression,
+				@Nullable MongoPersistentProperty sourceProperty) {
 
 			String rawPath = removePlaceholders(POSITIONAL_OPERATOR,
 					removePlaceholders(DOT_POSITIONAL_PATTERN, pathExpression));
 
+			if (sourceProperty != null && sourceProperty.getOwner().equals(entity)) {
+				return mappingContext
+						.getPersistentPropertyPath(PropertyPath.from(sourceProperty.getName(), entity.getTypeInformation()));
+			}
+
 			PropertyPath path = forName(rawPath);
 			if (path == null || isPathToJavaLangClassProperty(path)) {
 				return null;

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

@@ -46,6 +46,7 @@ import java.lang.annotation.Target;
  * @author Philipp Schneider
  * @author Johno Crawford
  * @author Christoph Strobl
+ * @author Dave Perryman
  */
 @Target({ ElementType.TYPE })
 @Documented
@@ -95,7 +96,8 @@ public @interface CompoundIndex {
 	boolean unique() default false;
 
 	/**
-	 * If set to true index will skip over any document that is missing the indexed field.
+	 * If set to true index will skip over any document that is missing the indexed field. <br />
+	 * Must not be used with {@link #partialFilter()}.
 	 *
 	 * @return {@literal false} by default.
 	 * @see <a href=
@@ -170,4 +172,14 @@ public @interface CompoundIndex {
 	 */
 	boolean background() default false;
 
+	/**
+	 * Only index the documents in a collection that meet a specified {@link IndexFilter filter expression}. <br />
+	 * Must not be used with {@link #sparse() sparse = true}.
+	 *
+	 * @return empty by default.
+	 * @see <a href=
+	 *      "https://docs.mongodb.com/manual/core/index-partial/">https://docs.mongodb.com/manual/core/index-partial/</a>
+	 * @since 3.1
+	 */
+	String partialFilter() default "";
 }

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

@@ -53,7 +53,8 @@ public @interface Indexed {
 	IndexDirection direction() default IndexDirection.ASCENDING;
 
 	/**
-	 * If set to true index will skip over any document that is missing the indexed field.
+	 * If set to true index will skip over any document that is missing the indexed field. <br />
+	 * Must not be used with {@link #partialFilter()}.
 	 *
 	 * @return {@literal false} by default.
 	 * @see <a href=
@@ -170,4 +171,15 @@ public @interface Indexed {
 	 * @since 2.2
 	 */
 	String expireAfter() default "";
+
+	/**
+	 * Only index the documents in a collection that meet a specified {@link IndexFilter filter expression}. <br />
+	 * Must not be used with {@link #sparse() sparse = true}.
+	 * 
+	 * @return empty by default.
+	 * @see <a href=
+	 *      "https://docs.mongodb.com/manual/core/index-partial/">https://docs.mongodb.com/manual/core/index-partial/</a>
+	 * @since 3.1
+	 */
+	String partialFilter() default "";
 }

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

@@ -15,10 +15,6 @@
  */
 package org.springframework.data.mongodb.core.index;
 
-import lombok.AccessLevel;
-import lombok.EqualsAndHashCode;
-import lombok.RequiredArgsConstructor;
-
 import java.time.Duration;
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -33,7 +29,6 @@ import java.util.stream.Collectors;
 
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
-
 import org.springframework.dao.InvalidDataAccessApiUsageException;
 import org.springframework.data.domain.Sort;
 import org.springframework.data.mapping.Association;
@@ -51,6 +46,7 @@ import org.springframework.data.mongodb.core.mapping.BasicMongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
+import org.springframework.data.mongodb.util.BsonUtils;
 import org.springframework.data.spel.EvaluationContextProvider;
 import org.springframework.data.util.TypeInformation;
 import org.springframework.expression.EvaluationContext;
@@ -74,6 +70,7 @@ import org.springframework.util.StringUtils;
  * @author Thomas Darimont
  * @author Martin Macko
  * @author Mark Paluch
+ * @author Dave Perryman
  * @since 1.5
  */
 public class MongoPersistentEntityIndexResolver implements IndexResolver {
@@ -385,6 +382,10 @@ public class MongoPersistentEntityIndexResolver implements IndexResolver {
 			indexDefinition.background();
 		}
 
+		if (StringUtils.hasText(index.partialFilter())) {
+			indexDefinition.partial(evaluatePartialFilter(index.partialFilter(), entity));
+		}
+
 		return new IndexDefinitionHolder(dotPath, indexDefinition, collection);
 	}
 
@@ -474,9 +475,25 @@ public class MongoPersistentEntityIndexResolver implements IndexResolver {
 			}
 		}
 
+		if (StringUtils.hasText(index.partialFilter())) {
+			indexDefinition.partial(evaluatePartialFilter(index.partialFilter(), persistentProperty.getOwner()));
+		}
+
 		return new IndexDefinitionHolder(dotPath, indexDefinition, collection);
 	}
 
+	private PartialIndexFilter evaluatePartialFilter(String filterExpression, PersistentEntity<?,?> entity) {
+
+		Object result = evaluate(filterExpression, getEvaluationContextForProperty(entity));
+
+		if (result instanceof org.bson.Document) {
+			return PartialIndexFilter.of((org.bson.Document) result);
+		}
+
+		return PartialIndexFilter.of(BsonUtils.parse(filterExpression, null));
+	}
+
+
 	/**
 	 * Creates {@link HashedIndex} wrapped in {@link IndexDefinitionHolder} out of {@link HashIndexed} for a given
 	 * {@link MongoPersistentProperty}.
@@ -733,8 +750,6 @@ public class MongoPersistentEntityIndexResolver implements IndexResolver {
 		 * @author Christoph Strobl
 		 * @author Mark Paluch
 		 */
-		@RequiredArgsConstructor(access = AccessLevel.PRIVATE)
-		@EqualsAndHashCode
 		static class Path {
 
 			private static final Path EMPTY = new Path(Collections.emptyList(), false);
@@ -742,6 +757,11 @@ public class MongoPersistentEntityIndexResolver implements IndexResolver {
 			private final List<PersistentProperty<?>> elements;
 			private final boolean cycle;
 
+			private Path(List<PersistentProperty<?>> elements, boolean cycle) {
+				this.elements = elements;
+				this.cycle = cycle;
+			}
+
 			/**
 			 * @return an empty {@link Path}.
 			 * @since 1.10.8
@@ -843,6 +863,28 @@ public class MongoPersistentEntityIndexResolver implements IndexResolver {
 
 				return builder.toString();
 			}
+
+			@Override
+			public boolean equals(Object o) {
+				if (this == o)
+					return true;
+				if (o == null || getClass() != o.getClass())
+					return false;
+
+				Path that = (Path) o;
+
+				if (this.cycle != that.cycle) {
+					return false;
+				}
+				return ObjectUtils.nullSafeEquals(this.elements, that.elements);
+			}
+
+			@Override
+			public int hashCode() {
+				int result = ObjectUtils.nullSafeHashCode(elements);
+				result = 31 * result + (cycle ? 1 : 0);
+				return result;
+			}
 		}
 	}
 

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

@@ -16,14 +16,8 @@
 package org.springframework.data.mongodb.core.index;
 
 import org.bson.Document;
-
-import lombok.AccessLevel;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-
 import org.springframework.data.mongodb.core.query.CriteriaDefinition;
-
-import com.mongodb.DBObject;
+import org.springframework.util.Assert;
 
 /**
  * {@link IndexFilter} implementation for usage with plain {@link Document} as well as {@link CriteriaDefinition} filter
@@ -32,10 +26,16 @@ import com.mongodb.DBObject;
  * @author Christoph Strobl
  * @since 1.10
  */
-@RequiredArgsConstructor(access = AccessLevel.PRIVATE)
 public class PartialIndexFilter implements IndexFilter {
 
-	private final @NonNull Object filterExpression;
+	private final Object filterExpression;
+
+	private PartialIndexFilter(Object filterExpression) {
+
+		Assert.notNull(filterExpression, "FilterExpression must not be null!");
+
+		this.filterExpression = filterExpression;
+	}
 
 	/**
 	 * Create new {@link PartialIndexFilter} for given {@link Document filter expression}.

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

@@ -164,7 +164,7 @@ public class BasicMongoPersistentEntity<T> extends BasicPersistentEntity<T, Mong
 	@Override
 	public org.springframework.data.mongodb.core.query.Collation getCollation() {
 
-		Object collationValue = collationExpression != null ? expression.getValue(getEvaluationContext(null), String.class)
+		Object collationValue = collationExpression != null ? collationExpression.getValue(getEvaluationContext(null), String.class)
 				: this.collation;
 
 		if (collationValue == null) {

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

@@ -48,6 +48,7 @@ import com.mongodb.client.model.geojson.Polygon;
  *
  * @author Oliver Gierke
  * @author Christoph Strobl
+ * @author Mark Paluch
  */
 public abstract class MongoSimpleTypes {
 
@@ -109,6 +110,10 @@ public abstract class MongoSimpleTypes {
 		@Override
 		public boolean isSimpleType(Class<?> type) {
 
+			if (type.isEnum()) {
+				return true;
+			}
+
 			if (type.getName().startsWith("java.time")) {
 				return false;
 			}

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/mapping/event/ReactiveAuditingEntityCallback.java

@@ -15,13 +15,12 @@
  */
 package org.springframework.data.mongodb.core.mapping.event;
 
-import reactor.core.publisher.Mono;
-
 import org.reactivestreams.Publisher;
+
 import org.springframework.beans.factory.ObjectFactory;
 import org.springframework.core.Ordered;
 import org.springframework.data.auditing.AuditingHandler;
-import org.springframework.data.auditing.IsNewAwareAuditingHandler;
+import org.springframework.data.auditing.ReactiveIsNewAwareAuditingHandler;
 import org.springframework.data.mapping.callback.EntityCallback;
 import org.springframework.data.mapping.context.MappingContext;
 import org.springframework.util.Assert;
@@ -34,7 +33,7 @@ import org.springframework.util.Assert;
  */
 public class ReactiveAuditingEntityCallback implements ReactiveBeforeConvertCallback<Object>, Ordered {
 
-	private final ObjectFactory<IsNewAwareAuditingHandler> auditingHandlerFactory;
+	private final ObjectFactory<ReactiveIsNewAwareAuditingHandler> auditingHandlerFactory;
 
 	/**
 	 * Creates a new {@link ReactiveAuditingEntityCallback} using the given {@link MappingContext} and
@@ -42,19 +41,19 @@ public class ReactiveAuditingEntityCallback implements ReactiveBeforeConvertCall
 	 *
 	 * @param auditingHandlerFactory must not be {@literal null}.
 	 */
-	public ReactiveAuditingEntityCallback(ObjectFactory<IsNewAwareAuditingHandler> auditingHandlerFactory) {
+	public ReactiveAuditingEntityCallback(ObjectFactory<ReactiveIsNewAwareAuditingHandler> auditingHandlerFactory) {
 
 		Assert.notNull(auditingHandlerFactory, "IsNewAwareAuditingHandler must not be null!");
 		this.auditingHandlerFactory = auditingHandlerFactory;
 	}
 
-	/* 
+	/*
 	 * (non-Javadoc)
 	 * @see org.springframework.data.mongodb.core.mapping.event.ReactiveBeforeConvertCallback#onBeforeConvert(java.lang.Object, java.lang.String)
 	 */
 	@Override
 	public Publisher<Object> onBeforeConvert(Object entity, String collection) {
-		return Mono.just(auditingHandlerFactory.getObject().markAudited(entity));
+		return auditingHandlerFactory.getObject().markAudited(entity);
 	}
 
 	/*

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core.messaging;
 
-import lombok.AllArgsConstructor;
-
 import java.time.Instant;
 import java.util.Arrays;
 import java.util.Collections;
@@ -64,7 +62,7 @@ import com.mongodb.client.model.changestream.FullDocument;
  */
 class ChangeStreamTask extends CursorReadingTask<ChangeStreamDocument<Document>, Object> {
 
-	private final Set<String> blacklist = new HashSet<>(
+	private final Set<String> denylist = new HashSet<>(
 			Arrays.asList("operationType", "fullDocument", "documentKey", "updateDescription", "ns"));
 
 	private final QueryMapper queryMapper;
@@ -178,7 +176,7 @@ class ChangeStreamTask extends CursorReadingTask<ChangeStreamDocument<Document>,
 							template.getConverter().getMappingContext(), queryMapper)
 					: Aggregation.DEFAULT_CONTEXT;
 
-			return agg.toPipeline(new PrefixingDelegatingAggregationOperationContext(context, "fullDocument", blacklist));
+			return agg.toPipeline(new PrefixingDelegatingAggregationOperationContext(context, "fullDocument", denylist));
 		}
 
 		if (filter instanceof List) {
@@ -217,12 +215,17 @@ class ChangeStreamTask extends CursorReadingTask<ChangeStreamDocument<Document>,
 	 *
 	 * @since 2.1
 	 */
-	@AllArgsConstructor
 	static class ChangeStreamEventMessage<T> implements Message<ChangeStreamDocument<Document>, T> {
 
 		private final ChangeStreamEvent<T> delegate;
 		private final MessageProperties messageProperties;
 
+		ChangeStreamEventMessage(ChangeStreamEvent<T> delegate, MessageProperties messageProperties) {
+
+			this.delegate = delegate;
+			this.messageProperties = messageProperties;
+		}
+
 		/*
 		 * (non-Javadoc)
 		 * @see org.springframework.data.mongodb.core.messaging.Message#getRaw()

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

@@ -15,10 +15,6 @@
  */
 package org.springframework.data.mongodb.core.messaging;
 
-import lombok.AccessLevel;
-import lombok.EqualsAndHashCode;
-import lombok.RequiredArgsConstructor;
-
 import java.time.Duration;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -34,6 +30,7 @@ import org.springframework.data.mongodb.core.messaging.SubscriptionRequest.Reque
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ErrorHandler;
+import org.springframework.util.ObjectUtils;
 
 /**
  * Simple {@link Executor} based {@link MessageListenerContainer} implementation for running {@link Task tasks} like
@@ -259,7 +256,6 @@ public class DefaultMessageListenerContainer implements MessageListenerContainer
 	 * @author Christoph Strobl
 	 * @since 2.1
 	 */
-	@EqualsAndHashCode
 	static class TaskSubscription implements Subscription {
 
 		private final Task task;
@@ -286,19 +282,39 @@ public class DefaultMessageListenerContainer implements MessageListenerContainer
 		public void cancel() throws DataAccessResourceFailureException {
 			task.cancel();
 		}
+
+		@Override
+		public boolean equals(Object o) {
+			if (this == o)
+				return true;
+			if (o == null || getClass() != o.getClass())
+				return false;
+
+			TaskSubscription that = (TaskSubscription) o;
+
+			return ObjectUtils.nullSafeEquals(this.task, that.task);
+		}
+
+		@Override
+		public int hashCode() {
+			return ObjectUtils.nullSafeHashCode(task);
+		}
 	}
 
 	/**
 	 * @author Christoph Strobl
 	 * @since 2.1
 	 */
-	@RequiredArgsConstructor(access = AccessLevel.PACKAGE)
 	private static class DecoratingLoggingErrorHandler implements ErrorHandler {
 
 		private final Log logger = LogFactory.getLog(DecoratingLoggingErrorHandler.class);
 
 		private final ErrorHandler delegate;
 
+		DecoratingLoggingErrorHandler(ErrorHandler delegate) {
+			this.delegate = delegate;
+		}
+
 		@Override
 		public void handleError(Throwable t) {
 

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

@@ -15,8 +15,6 @@
  */
 package org.springframework.data.mongodb.core.messaging;
 
-import lombok.ToString;
-
 import org.bson.Document;
 import org.springframework.data.mongodb.core.convert.MongoConverter;
 import org.springframework.util.ClassUtils;
@@ -26,7 +24,6 @@ import org.springframework.util.ClassUtils;
  * @author Mark Paluch
  * @since 2.1
  */
-@ToString(of = { "delegate", "targetType" })
 class LazyMappingDelegatingMessage<S, T> implements Message<S, T> {
 
 	private final Message<S, ?> delegate;
@@ -82,4 +79,8 @@ class LazyMappingDelegatingMessage<S, T> implements Message<S, T> {
 	public MessageProperties getProperties() {
 		return delegate.getProperties();
 	}
+
+	public String toString() {
+		return "LazyMappingDelegatingMessage(delegate=" + this.delegate + ", targetType=" + this.targetType + ")";
+	}
 }

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

@@ -15,11 +15,9 @@
  */
 package org.springframework.data.mongodb.core.messaging;
 
-import lombok.EqualsAndHashCode;
-import lombok.ToString;
-
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 
 /**
  * General message abstraction for any type of Event / Message published by MongoDB server to the client. This might be
@@ -65,8 +63,6 @@ public interface Message<S, T> {
 	 * @author Christoph Strobl
 	 * @since 2.1
 	 */
-	@ToString
-	@EqualsAndHashCode
 	class MessageProperties {
 
 		private static final MessageProperties EMPTY = new MessageProperties();
@@ -111,6 +107,34 @@ public interface Message<S, T> {
 			return new MessagePropertiesBuilder();
 		}
 
+		@Override
+		public boolean equals(Object o) {
+			if (this == o)
+				return true;
+			if (o == null || getClass() != o.getClass())
+				return false;
+
+			MessageProperties that = (MessageProperties) o;
+
+			if (!ObjectUtils.nullSafeEquals(this.databaseName, that.databaseName)) {
+				return false;
+			}
+
+			return ObjectUtils.nullSafeEquals(this.collectionName, that.collectionName);
+		}
+
+		@Override
+		public int hashCode() {
+			int result = ObjectUtils.nullSafeHashCode(databaseName);
+			result = 31 * result + ObjectUtils.nullSafeHashCode(collectionName);
+			return result;
+		}
+
+		public String toString() {
+			return "Message.MessageProperties(databaseName=" + this.getDatabaseName() + ", collectionName="
+					+ this.getCollectionName() + ")";
+		}
+
 		/**
 		 * Builder for {@link MessageProperties}.
 		 *

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

@@ -15,11 +15,9 @@
  */
 package org.springframework.data.mongodb.core.messaging;
 
-import lombok.EqualsAndHashCode;
-import lombok.ToString;
-
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 
 /**
  * Trivial {@link Message} implementation.
@@ -27,8 +25,6 @@ import org.springframework.util.Assert;
  * @author Christoph Strobl
  * @since 2.1
  */
-@EqualsAndHashCode
-@ToString
 class SimpleMessage<S, T> implements Message<S, T> {
 
 	private @Nullable final S raw;
@@ -75,4 +71,35 @@ class SimpleMessage<S, T> implements Message<S, T> {
 	public MessageProperties getProperties() {
 		return properties;
 	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (this == o)
+			return true;
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		SimpleMessage<?, ?> that = (SimpleMessage<?, ?>) o;
+
+		if (!ObjectUtils.nullSafeEquals(this.raw, that.raw)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(this.body, that.body)) {
+			return false;
+		}
+		return ObjectUtils.nullSafeEquals(this.properties, that.properties);
+	}
+
+	@Override
+	public int hashCode() {
+		int result = ObjectUtils.nullSafeHashCode(raw);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(body);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(properties);
+		return result;
+	}
+
+	public String toString() {
+		return "SimpleMessage(raw=" + this.getRaw() + ", body=" + this.getBody() + ", properties=" + this.getProperties()
+				+ ")";
+	}
 }

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

@@ -67,6 +67,7 @@ public class BasicUpdate extends Update {
 	}
 
 	@Override
+	@Deprecated
 	public Update pushAll(String key, Object[] values) {
 		Document keyValue = new Document();
 		keyValue.put(key, values);

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

@@ -15,11 +15,6 @@
  */
 package org.springframework.data.mongodb.core.query;
 
-import lombok.AccessLevel;
-import lombok.AllArgsConstructor;
-import lombok.Getter;
-import lombok.RequiredArgsConstructor;
-
 import java.util.Locale;
 import java.util.Optional;
 
@@ -515,8 +510,6 @@ public class Collation {
 	 *
 	 * @since 2.0
 	 */
-	@AllArgsConstructor(access = AccessLevel.PACKAGE)
-	@Getter
 	static class ICUComparisonLevel implements ComparisonLevel {
 
 		private final int level;
@@ -526,6 +519,24 @@ public class Collation {
 		ICUComparisonLevel(int level) {
 			this(level, Optional.empty(), Optional.empty());
 		}
+
+		ICUComparisonLevel(int level, Optional<CaseFirst> caseFirst, Optional<Boolean> caseLevel) {
+			this.level = level;
+			this.caseFirst = caseFirst;
+			this.caseLevel = caseLevel;
+		}
+
+		public int getLevel() {
+			return this.level;
+		}
+
+		public Optional<CaseFirst> getCaseFirst() {
+			return this.caseFirst;
+		}
+
+		public Optional<Boolean> getCaseLevel() {
+			return this.caseLevel;
+		}
 	}
 
 	/**
@@ -650,7 +661,6 @@ public class Collation {
 	/**
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor(access = AccessLevel.PRIVATE)
 	public static class CaseFirst {
 
 		private static final CaseFirst UPPER = new CaseFirst("upper");
@@ -659,6 +669,10 @@ public class Collation {
 
 		private final String state;
 
+		private CaseFirst(String state) {
+			this.state = state;
+		}
+
 		/**
 		 * Sort uppercase before lowercase.
 		 *
@@ -690,7 +704,6 @@ public class Collation {
 	/**
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor(access = AccessLevel.PACKAGE)
 	public static class Alternate {
 
 		private static final Alternate NON_IGNORABLE = new Alternate("non-ignorable", Optional.empty());
@@ -698,6 +711,11 @@ public class Collation {
 		final String alternate;
 		final Optional<String> maxVariable;
 
+		Alternate(String alternate, Optional<String> maxVariable) {
+			this.alternate = alternate;
+			this.maxVariable = maxVariable;
+		}
+
 		/**
 		 * Consider Whitespace and punctuation as base characters.
 		 *
@@ -761,12 +779,17 @@ public class Collation {
 	 * @see <a href="http://site.icu-project.org">ICU - International Components for Unicode</a>
 	 * @since 2.0
 	 */
-	@RequiredArgsConstructor(access = AccessLevel.PRIVATE)
 	public static class CollationLocale {
 
 		private final String language;
 		private final Optional<String> variant;
 
+		private CollationLocale(String language, Optional<String> variant) {
+
+			this.language = language;
+			this.variant = variant;
+		}
+
 		/**
 		 * Create new {@link CollationLocale} for given language.
 		 *

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

@@ -15,54 +15,134 @@
  */
 package org.springframework.data.mongodb.core.query;
 
-import lombok.EqualsAndHashCode;
-
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Map.Entry;
 
 import org.bson.Document;
+
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
+import org.springframework.util.ObjectUtils;
 
 /**
+ * Field projection.
+ *
  * @author Thomas Risberg
  * @author Oliver Gierke
  * @author Patryk Wasik
  * @author Christoph Strobl
  * @author Mark Paluch
+ * @author Owen Q
  */
-@EqualsAndHashCode
 public class Field {
 
-	private final Map<String, Integer> criteria = new HashMap<String, Integer>();
-	private final Map<String, Object> slices = new HashMap<String, Object>();
-	private final Map<String, Criteria> elemMatchs = new HashMap<String, Criteria>();
+	private final Map<String, Integer> criteria = new HashMap<>();
+	private final Map<String, Object> slices = new HashMap<>();
+	private final Map<String, Criteria> elemMatchs = new HashMap<>();
 	private @Nullable String positionKey;
 	private int positionValue;
 
-	public Field include(String key) {
-		criteria.put(key, Integer.valueOf(1));
+	/**
+	 * Include a single {@code field} to be returned by the query operation.
+	 *
+	 * @param field the document field name to be included.
+	 * @return {@code this} field projection instance.
+	 */
+	public Field include(String field) {
+
+		Assert.notNull(field, "Key must not be null!");
+
+		criteria.put(field, 1);
+
 		return this;
 	}
 
-	public Field exclude(String key) {
-		criteria.put(key, Integer.valueOf(0));
+	/**
+	 * Include one or more {@code fields} to be returned by the query operation.
+	 *
+	 * @param fields the document field names to be included.
+	 * @return {@code this} field projection instance.
+	 * @since 3.1
+	 */
+	public Field include(String... fields) {
+
+		Assert.notNull(fields, "Keys must not be null!");
+
+		for (String key : fields) {
+			criteria.put(key, 1);
+		}
+
 		return this;
 	}
 
-	public Field slice(String key, int size) {
-		slices.put(key, Integer.valueOf(size));
+	/**
+	 * Exclude a single {@code field} from being returned by the query operation.
+	 *
+	 * @param field the document field name to be included.
+	 * @return {@code this} field projection instance.
+	 */
+	public Field exclude(String field) {
+
+		Assert.notNull(field, "Key must not be null!");
+
+		criteria.put(field, 0);
+
 		return this;
 	}
 
-	public Field slice(String key, int offset, int size) {
-		slices.put(key, new Integer[] { Integer.valueOf(offset), Integer.valueOf(size) });
+	/**
+	 * Exclude one or more {@code fields} from being returned by the query operation.
+	 *
+	 * @param fields the document field names to be included.
+	 * @return {@code this} field projection instance.
+	 * @since 3.1
+	 */
+	public Field exclude(String... fields) {
+
+		Assert.notNull(fields, "Keys must not be null!");
+
+		for (String key : fields) {
+			criteria.put(key, 0);
+		}
+
 		return this;
 	}
 
-	public Field elemMatch(String key, Criteria elemMatchCriteria) {
-		elemMatchs.put(key, elemMatchCriteria);
+	/**
+	 * Project a {@code $slice} of the array {@code field} using the first {@code size} elements.
+	 *
+	 * @param field the document field name to project, must be an array field.
+	 * @param size the number of elements to include.
+	 * @return {@code this} field projection instance.
+	 */
+	public Field slice(String field, int size) {
+
+		Assert.notNull(field, "Key must not be null!");
+
+		slices.put(field, size);
+
+		return this;
+	}
+
+	/**
+	 * Project a {@code $slice} of the array {@code field} using the first {@code size} elements starting at
+	 * {@code offset}.
+	 *
+	 * @param field the document field name to project, must be an array field.
+	 * @param offset the offset to start at.
+	 * @param size the number of elements to include.
+	 * @return {@code this} field projection instance.
+	 */
+	public Field slice(String field, int offset, int size) {
+
+		slices.put(field, new Integer[] { offset, size });
+		return this;
+	}
+
+	public Field elemMatch(String field, Criteria elemMatchCriteria) {
+
+		elemMatchs.put(field, elemMatchCriteria);
 		return this;
 	}
 
@@ -72,7 +152,7 @@ public class Field {
 	 *
 	 * @param field query array field, must not be {@literal null} or empty.
 	 * @param value
-	 * @return
+	 * @return {@code this} field projection instance.
 	 */
 	public Field position(String field, int value) {
 
@@ -103,4 +183,40 @@ public class Field {
 
 		return document;
 	}
+
+	@Override
+	public boolean equals(Object o) {
+
+		if (this == o)
+			return true;
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		Field field = (Field) o;
+
+		if (positionValue != field.positionValue) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(criteria, field.criteria)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(slices, field.slices)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(elemMatchs, field.elemMatchs)) {
+			return false;
+		}
+		return ObjectUtils.nullSafeEquals(positionKey, field.positionKey);
+	}
+
+	@Override
+	public int hashCode() {
+
+		int result = ObjectUtils.nullSafeHashCode(criteria);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(slices);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(elemMatchs);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(positionKey);
+		result = 31 * result + positionValue;
+		return result;
+	}
 }

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

@@ -284,9 +284,21 @@ public class Meta {
 		 */
 		EXHAUST,
 
-		/** Allows querying of a replica slave. */
+		/**
+		 * Allows querying of a replica.
+		 *
+		 * @deprecated since 3.0.2, use {@link #SECONDARY_READS} instead.
+		 */
+		@Deprecated
 		SLAVE_OK,
 
+		/**
+		 * Allows querying of a replica.
+		 *
+		 * @since 3.0.2
+		 */
+		SECONDARY_READS,
+
 		/**
 		 * Sets the cursor to return partial data from a query against a sharded cluster in which some shards do not respond
 		 * rather than throwing an error.

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

@@ -411,18 +411,33 @@ public class Query {
 	}
 
 	/**
-	 * Allows querying of a replica slave.
+	 * Allows querying of a replica.
 	 *
 	 * @return this.
 	 * @see org.springframework.data.mongodb.core.query.Meta.CursorOption#SLAVE_OK
 	 * @since 1.10
+	 * @deprecated since 3.0.2, use {@link #allowSecondaryReads()}.
 	 */
+	@Deprecated
 	public Query slaveOk() {
 
 		meta.addFlag(Meta.CursorOption.SLAVE_OK);
 		return this;
 	}
 
+	/**
+	 * Allows querying of a replica.
+	 *
+	 * @return this.
+	 * @see org.springframework.data.mongodb.core.query.Meta.CursorOption#SECONDARY_READS
+	 * @since 3.0.2
+	 */
+	public Query allowSecondaryReads() {
+
+		meta.addFlag(Meta.CursorOption.SECONDARY_READS);
+		return this;
+	}
+
 	/**
 	 * @return this.
 	 * @see org.springframework.data.mongodb.core.query.Meta.CursorOption#PARTIAL

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

@@ -15,14 +15,10 @@
  */
 package org.springframework.data.mongodb.core.query;
 
-import lombok.AccessLevel;
-import lombok.EqualsAndHashCode;
-import lombok.NonNull;
-import lombok.RequiredArgsConstructor;
-
 import java.util.Set;
 
 import org.springframework.data.domain.ExampleMatcher;
+import org.springframework.util.ObjectUtils;
 
 /**
  * {@link ExampleMatcher} implementation for query by example (QBE). Unlike plain {@link ExampleMatcher} this untyped
@@ -33,11 +29,13 @@ import org.springframework.data.domain.ExampleMatcher;
  * @author Mark Paluch
  * @since 2.0
  */
-@EqualsAndHashCode
-@RequiredArgsConstructor(access = AccessLevel.PRIVATE)
 public class UntypedExampleMatcher implements ExampleMatcher {
 
-	private final @NonNull ExampleMatcher delegate;
+	private final ExampleMatcher delegate;
+
+	private UntypedExampleMatcher(ExampleMatcher delegate) {
+		this.delegate = delegate;
+	}
 
 	/*
 	 * (non-Javadoc)
@@ -224,4 +222,21 @@ public class UntypedExampleMatcher implements ExampleMatcher {
 		return delegate.getMatchMode();
 	}
 
+	@Override
+	public boolean equals(Object o) {
+
+		if (this == o)
+			return true;
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		UntypedExampleMatcher that = (UntypedExampleMatcher) o;
+
+		return ObjectUtils.nullSafeEquals(delegate, that.delegate);
+	}
+
+	@Override
+	public int hashCode() {
+		return ObjectUtils.nullSafeHashCode(delegate);
+	}
 }

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

@@ -15,10 +15,8 @@
  */
 package org.springframework.data.mongodb.core.schema;
 
-import lombok.AllArgsConstructor;
-import lombok.NonNull;
-
 import org.bson.Document;
+import org.springframework.util.Assert;
 
 /**
  * Value object representing a MongoDB-specific JSON schema which is the default {@link MongoJsonSchema} implementation.
@@ -27,10 +25,15 @@ import org.bson.Document;
  * @author Mark Paluch
  * @since 2.1
  */
-@AllArgsConstructor
 class DefaultMongoJsonSchema implements MongoJsonSchema {
 
-	private final @NonNull JsonSchemaObject root;
+	private final JsonSchemaObject root;
+
+	DefaultMongoJsonSchema(JsonSchemaObject root) {
+		
+		Assert.notNull(root, "Root must not be null!");
+		this.root = root;
+	}
 
 	/*
 	 * (non-Javadoc)

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

@@ -15,10 +15,8 @@
  */
 package org.springframework.data.mongodb.core.schema;
 
-import lombok.AllArgsConstructor;
-import lombok.NonNull;
-
 import org.bson.Document;
+import org.springframework.util.Assert;
 
 /**
  * JSON schema backed by a {@link org.bson.Document} object.
@@ -26,10 +24,15 @@ import org.bson.Document;
  * @author Mark Paluch
  * @since 2.1
  */
-@AllArgsConstructor
 class DocumentJsonSchema implements MongoJsonSchema {
 
-	private final @NonNull Document document;
+	private final Document document;
+
+	DocumentJsonSchema(Document document) {
+		
+		Assert.notNull(document, "Document must not be null!");
+		this.document = document;
+	}
 
 	/*
 	 * (non-Javadoc)