switch to commons branch

Reformat code. Merge if-statements.

Original pull request: #889.

Jenkinsfile

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

pom.xml

@@ -5,7 +5,7 @@
 
 	<groupId>org.springframework.data</groupId>
 	<artifactId>spring-data-mongodb-parent</artifactId>
-	<version>3.1.0-RC1</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>2.4.0-RC1</version>
+		<version>2.4.0-SNAPSHOT</version>
 	</parent>
 
 	<modules>
@@ -26,7 +26,7 @@
 	<properties>
 		<project.type>multi</project.type>
 		<dist.id>spring-data-mongodb</dist.id>
-		<springdata.commons>2.4.0-RC1</springdata.commons>
+		<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>
@@ -134,8 +134,8 @@
 
 	<repositories>
 		<repository>
-			<id>spring-libs-milestone</id>
-			<url>https://repo.spring.io/libs-milestone</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>3.1.0-RC1</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>3.1.0-RC1</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>3.1.0-RC1</version>
+		<version>3.1.0-STATIC-METADATA-SNAPSHOT</version>
 		<relativePath>../pom.xml</relativePath>
 	</parent>
 

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

@@ -28,6 +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;
@@ -75,12 +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/MongoTemplate.java

@@ -1977,9 +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)

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/AggregationPipeline.java

@@ -18,6 +18,7 @@ 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;
@@ -26,6 +27,7 @@ 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 {
@@ -45,6 +47,8 @@ public class AggregationPipeline {
 	 * @param aggregationOperations must not be {@literal null}.
 	 */
 	public AggregationPipeline(List<AggregationOperation> aggregationOperations) {
+
+		Assert.notNull(aggregationOperations, "AggregationOperations must not be null!");
 		pipeline = new ArrayList<>(aggregationOperations);
 	}
 
@@ -82,30 +86,77 @@ public class AggregationPipeline {
 	 */
 	public boolean isOutOrMerge() {
 
-		if (pipeline.isEmpty()) {
+		if (isEmpty()) {
 			return false;
 		}
 
-		String operator = pipeline.get(pipeline.size() - 1).getOperator();
-		return operator.equals("$out") || operator.equals("$merge");
+		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 aggregationOperation : pipeline) {
+		for (AggregationOperation operation : pipeline) {
 
-			if (aggregationOperation instanceof OutOperation && !isLast(aggregationOperation)) {
+			if (isOut(operation) && !isLast(operation)) {
 				throw new IllegalArgumentException("The $out operator must be the last stage in the pipeline.");
 			}
 
-			if (aggregationOperation instanceof MergeOperation && !isLast(aggregationOperation)) {
+			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/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/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/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/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/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/query/Field.java

@@ -20,47 +20,129 @@ 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
  */
 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;
 	}
 
@@ -70,7 +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) {
 

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/repository/query/ReactiveMongoQueryMethod.java

@@ -94,8 +94,8 @@ public class ReactiveMongoQueryMethod extends MongoQueryMethod {
 		}
 
 		this.method = method;
-		this.isCollectionQuery = Lazy.of(() -> !(isPageQuery() || isSliceQuery())
-				&& ReactiveWrappers.isMultiValueType(metadata.getReturnType(method).getType()));
+		this.isCollectionQuery = Lazy.of(() -> (!(isPageQuery() || isSliceQuery())
+				&& ReactiveWrappers.isMultiValueType(metadata.getReturnType(method).getType()) || super.isCollectionQuery()));
 	}
 
 	/*

spring-data-mongodb/src/main/java/org/springframework/data/mongodb/util/json/ParameterBindingDocumentCodec.java

@@ -365,15 +365,7 @@ public class ParameterBindingDocumentCodec implements CollectibleCodec<Document>
 		reader.readStartArray();
 		List<Object> list = new ArrayList<>();
 		while (reader.readBsonType() != BsonType.END_OF_DOCUMENT) {
-
-			// Spring Data Customization START
-			Object listValue = readValue(reader, decoderContext);
-			if (listValue instanceof Collection) {
-				list.addAll((Collection) listValue);
-				break;
-			}
-			list.add(listValue);
-			// Spring Data Customization END
+			list.add(readValue(reader, decoderContext));
 		}
 		reader.readEndArray();
 		return list;

spring-data-mongodb/src/main/kotlin/org/springframework/data/mongodb/core/query/KPropertyPath.kt

@@ -20,8 +20,10 @@ import kotlin.reflect.KProperty1
 
 /**
  * Abstraction of a property path consisting of [KProperty].
+ *
  * @author Tjeu Kayim
  * @author Mark Paluch
+ * @author Yoann de Martino
  * @since 2.2
  */
 class KPropertyPath<T, U>(
@@ -45,7 +47,7 @@ internal fun asString(property: KProperty<*>): String {
  * Builds [KPropertyPath] from Property References.
  * Refer to a field in an embedded/nested document.
  *
- * For example, referring to the field "book.author":
+ * For example, referring to the field "author.name":
  * ```
  * Book::author / Author::name isEqualTo "Herman Melville"
  * ```

spring-data-mongodb/src/main/kotlin/org/springframework/data/mongodb/core/query/KPropertyPathExtensions.kt

@@ -0,0 +1,26 @@
+/*
+ * 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.query
+
+import kotlin.reflect.KProperty
+
+/**
+ * Extension for [KProperty] providing an `toPath` function to render a [KProperty] as property path.
+ *
+ * @author Mark Paluch
+ * @since 3.1
+ */
+fun KProperty<*>.toPath(): String = asString(this)

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/buildtimetypeinfo/Address.java

@@ -0,0 +1,68 @@
+/*
+ * 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.buildtimetypeinfo;
+
+import org.springframework.util.ObjectUtils;
+
+/**
+ * @author Christoph Strobl
+ * @since 2020/10
+ */
+public class Address {
+
+	String city;
+	String street;
+
+	public Address(String city, String street) {
+		this.city = city;
+		this.street = street;
+	}
+
+	public String getCity() {
+		return city;
+	}
+
+	public String getStreet() {
+		return street;
+	}
+
+	@Override
+	public String toString() {
+		return "Address{" + "city='" + city + '\'' + ", street='" + street + '\'' + '}';
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (this == o)
+			return true;
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		Address address = (Address) o;
+
+		if (!ObjectUtils.nullSafeEquals(city, address.city)) {
+			return false;
+		}
+		return ObjectUtils.nullSafeEquals(street, address.street);
+	}
+
+	@Override
+	public int hashCode() {
+		int result = ObjectUtils.nullSafeHashCode(city);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(street);
+		return result;
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/buildtimetypeinfo/AddressTypeInformation.java

@@ -0,0 +1,50 @@
+/*
+ * 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.buildtimetypeinfo;
+
+import org.springframework.data.mapping.model.DomainTypeConstructor;
+import org.springframework.data.mapping.model.DomainTypeInformation;
+import org.springframework.data.mapping.model.Field;
+
+/**
+ * @author Christoph Strobl
+ * @since 2020/10
+ */
+public class AddressTypeInformation extends DomainTypeInformation<Address> {
+
+	private static final AddressTypeInformation INSTANCE = new AddressTypeInformation();
+
+	private AddressTypeInformation() {
+
+		super(Address.class);
+
+		// CONSTRUCTOR
+		setConstructor(computePreferredConstructor());
+
+		// FIELDS
+		addField(Field.<Address> string("city").getter(Address::getCity));
+		addField(Field.<Address> string("street").getter(Address::getStreet));
+	}
+
+	public static AddressTypeInformation instance() {
+		return INSTANCE;
+	}
+
+	private DomainTypeConstructor<Address> computePreferredConstructor() {
+		return DomainTypeConstructor.<Address> builder().args("city", "street")
+				.newInstanceFunction(args -> new Address((String) args[0], (String) args[1]));
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/buildtimetypeinfo/Person.java

@@ -0,0 +1,139 @@
+/*
+ * 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.buildtimetypeinfo;
+
+import java.util.List;
+
+import org.springframework.util.ObjectUtils;
+
+/**
+ * @author Christoph Strobl
+ * @since 2020/10
+ */
+public class Person {
+
+	private long id;
+	private String firstname, lastname; // TODO: we need a persistence constructor to resolve this here.
+	private int age;
+	private Address address;
+	private List<String> nicknames;
+
+	public Person(String firstname, String lastname) {
+		this.firstname = firstname;
+		this.lastname = lastname;
+	}
+
+	private Person(long id, String firstname, String lastname, int age, Address address, List<String> nicknames) {
+		this.id = id;
+		this.firstname = firstname;
+		this.lastname = lastname;
+		this.age = age;
+		this.address = address;
+		this.nicknames = nicknames;
+	}
+
+	public String getFirstname() {
+		return firstname;
+	}
+
+	public String getLastname() {
+		return lastname;
+	}
+
+	public int getAge() {
+		return age;
+	}
+
+	public void setAge(int age) {
+		this.age = age;
+	}
+
+	public long getId() {
+		return id;
+	}
+
+	public Person withId(long id) {
+
+		return new Person(id, firstname, lastname, age, address, nicknames);
+
+	}
+
+	public Address getAddress() {
+		return address;
+	}
+
+	public void setAddress(Address address) {
+		this.address = address;
+	}
+
+	public List<String> getNicknames() {
+		return nicknames;
+	}
+
+	public void setNicknames(List<String> nicknames) {
+		this.nicknames = nicknames;
+	}
+
+	public void setFirstname(String firstname) {
+		this.firstname = firstname;
+	}
+
+	public void setLastname(String lastname) {
+		this.lastname = lastname;
+	}
+
+	@Override
+	public String toString() {
+		return "Person{" + "id=" + id + ", firstname='" + firstname + '\'' + ", lastname='" + lastname + '\'' + ", age="
+				+ age + ", address=" + address + ", nicknames=" + nicknames + '}';
+	}
+
+	@Override
+	public boolean equals(Object o) {
+		if (this == o)
+			return true;
+		if (o == null || getClass() != o.getClass())
+			return false;
+
+		Person person = (Person) o;
+
+		if (id != person.id)
+			return false;
+		if (age != person.age)
+			return false;
+		if (!ObjectUtils.nullSafeEquals(firstname, person.firstname)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(lastname, person.lastname)) {
+			return false;
+		}
+		if (!ObjectUtils.nullSafeEquals(address, person.address)) {
+			return false;
+		}
+		return ObjectUtils.nullSafeEquals(nicknames, person.nicknames);
+	}
+
+	@Override
+	public int hashCode() {
+		int result = (int) (id ^ (id >>> 32));
+		result = 31 * result + ObjectUtils.nullSafeHashCode(firstname);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(lastname);
+		result = 31 * result + age;
+		result = 31 * result + ObjectUtils.nullSafeHashCode(address);
+		result = 31 * result + ObjectUtils.nullSafeHashCode(nicknames);
+		return result;
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/buildtimetypeinfo/PersonTypeInformation.java

@@ -0,0 +1,128 @@
+/*
+ * 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.buildtimetypeinfo;
+
+import java.lang.annotation.Annotation;
+import java.util.List;
+
+import org.springframework.data.mapping.model.DomainTypeConstructor;
+import org.springframework.data.mapping.model.DomainTypeInformation;
+import org.springframework.data.mapping.model.Field;
+import org.springframework.data.mapping.model.ListTypeInformation;
+import org.springframework.data.mapping.model.StringTypeInformation;
+import org.springframework.data.mongodb.core.mapping.Document;
+import org.springframework.data.mongodb.core.mapping.FieldType;
+
+/**
+ * @author Christoph Strobl
+ * @since 2020/10
+ */
+public class PersonTypeInformation extends DomainTypeInformation<Person> {
+
+	private static final PersonTypeInformation INSTANCE = new PersonTypeInformation();
+
+	private PersonTypeInformation() {
+
+		super(Person.class);
+
+		// CONSTRUCTOR
+		setConstructor(computePreferredConstructor());
+
+		// ANNOTATIONS
+		addAnnotation(computeAtDocumentAnnotation());
+
+		// FIELDS
+		addField(
+				Field.<Person> int64("id").annotatedWithAtId().getter(Person::getId).wither((bean, id) -> bean.withId(id)));
+		addField(Field.<Person> string("firstname").getter(Person::getFirstname).annotation(atFieldOnFirstname()));
+		addField(Field.<Person> string("lastname").getter(Person::getLastname));
+		addField(Field.<Person> int32("age").getter(Person::getAge).setter(Person::setAge));
+		addField(Field.<Person, Address> type("address", AddressTypeInformation.instance()).getter(Person::getAddress)
+				.setter(Person::setAddress));
+		addField(Field.<Person, List<String>> type("nicknames", new ListTypeInformation<>(StringTypeInformation.instance()))
+				.getter(Person::getNicknames).setter(Person::setNicknames));
+
+	}
+
+	public static PersonTypeInformation instance() {
+		return INSTANCE;
+	}
+
+	private DomainTypeConstructor<Person> computePreferredConstructor() {
+		return DomainTypeConstructor.<Person> builder().args("firstname", "lastname")
+				.newInstanceFunction((args) -> new Person((String) args[0], (String) args[1]));
+	}
+
+	private Document computeAtDocumentAnnotation() {
+		return new Document() {
+			@Override
+			public Class<? extends Annotation> annotationType() {
+				return Document.class;
+			}
+
+			@Override
+			public String value() {
+				return collection();
+			}
+
+			@Override
+			public String collection() {
+				return "star-wars";
+			}
+
+			@Override
+			public String language() {
+				return "";
+			}
+
+			@Override
+			public String collation() {
+				return "";
+			}
+		};
+	}
+
+	private Annotation atFieldOnFirstname() {
+
+		return new org.springframework.data.mongodb.core.mapping.Field() {
+
+			@Override
+			public Class<? extends Annotation> annotationType() {
+				return org.springframework.data.mongodb.core.mapping.Field.class;
+			}
+
+			@Override
+			public String value() {
+				return "first-name";
+			}
+
+			@Override
+			public String name() {
+				return value();
+			}
+
+			@Override
+			public int order() {
+				return 0;
+			}
+
+			@Override
+			public FieldType targetType() {
+				return FieldType.IMPLICIT;
+			}
+		};
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/MongoTemplateTests.java

@@ -25,7 +25,7 @@ import lombok.Data;
 import lombok.EqualsAndHashCode;
 import lombok.NoArgsConstructor;
 import lombok.Value;
-import lombok.experimental.Wither;
+import lombok.With;
 
 import java.lang.reflect.InvocationTargetException;
 import java.math.BigDecimal;
@@ -1839,7 +1839,7 @@ public class MongoTemplateTests {
 		assertThat(result.property3).isEqualTo(obj.property3);
 	}
 
-	@Test // DATAMONGO-702
+	@Test // DATAMONGO-702, DATAMONGO-2294
 	public void queryShouldSupportRealAndAliasedPropertyNamesForFieldExclusions() {
 
 		ObjectWith3AliasedFields obj = new ObjectWith3AliasedFields();
@@ -1852,8 +1852,7 @@ public class MongoTemplateTests {
 
 		Query query = new Query(Criteria.where("id").is(obj.id));
 		query.fields() //
-				.exclude("property2") // real property name
-				.exclude("prop3"); // aliased property name
+				.exclude("property2", "prop3"); // real property name, aliased property name
 
 		ObjectWith3AliasedFields result = template.findOne(query, ObjectWith3AliasedFields.class);
 
@@ -3669,6 +3668,27 @@ public class MongoTemplateTests {
 		assertThat(target.inner.id).isEqualTo(innerId);
 	}
 
+	@Test // DATAMONGO-2294
+	public void shouldProjectWithCollections() {
+
+		MyPerson person = new MyPerson("Walter");
+		person.address = new Address("TX", "Austin");
+		template.save(person);
+
+		Query queryByChainedInclude = query(where("name").is("Walter"));
+		queryByChainedInclude.fields().include("id").include("name");
+
+		Query queryByCollectionInclude = query(where("name").is("Walter"));
+		queryByCollectionInclude.fields().include("id", "name");
+
+		MyPerson first = template.findAndReplace(queryByChainedInclude, new MyPerson("Walter"));
+		MyPerson second = template.findAndReplace(queryByCollectionInclude, new MyPerson("Walter"));
+
+		assertThat(first).isEqualTo(second);
+		assertThat(first.address).isNull();
+		assertThat(second.address).isNull();
+	}
+
 	@Test // DATAMONGO-2451
 	public void sortOnIdFieldWithExplicitTypeShouldWork() {
 
@@ -4180,7 +4200,7 @@ public class MongoTemplateTests {
 	// DATAMONGO-1992
 
 	@AllArgsConstructor
-	@Wither
+	@With
 	static class ImmutableVersioned {
 
 		final @Id String id;
@@ -4193,7 +4213,7 @@ public class MongoTemplateTests {
 	}
 
 	@Value
-	@Wither
+	@With
 	static class ImmutableAudited {
 		@Id String id;
 		@LastModifiedDate Instant modified;

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ScriptOperatorsUnitTests.java

@@ -0,0 +1,96 @@
+/*
+ * 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 static org.assertj.core.api.Assertions.*;
+import static org.springframework.data.mongodb.core.aggregation.ScriptOperators.*;
+
+import java.util.Collections;
+
+import org.bson.Document;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Unit tests for {@link ScriptOperators}.
+ *
+ * @author Christoph Strobl
+ */
+class ScriptOperatorsUnitTests {
+
+	private static final String FUNCTION_BODY = "function(name) { return hex_md5(name) == \"15b0a220baa16331e8d80e15367677ad\" }";
+	private static final Document EMPTY_ARGS_FUNCTION_DOCUMENT = new Document("body", FUNCTION_BODY)
+			.append("args", Collections.emptyList()).append("lang", "js");
+
+	private static final String INIT_FUNCTION = "function() { return { count: 0, sum: 0 } }";
+	private static final String ACC_FUNCTION = "function(state, numCopies) { return { count: state.count + 1, sum: state.sum + numCopies } }";
+	private static final String MERGE_FUNCTION = "function(state1, state2) { return { count: state1.count + state2.count, sum: state1.sum + state2.sum } }";
+	private static final String FINALIZE_FUNCTION = "function(state) { return (state.sum / state.count) }";
+
+	private static final Document $ACCUMULATOR = Document.parse("{" + //
+			"      $accumulator:" + //
+			"      {" + //
+			"        init: '" + INIT_FUNCTION + "'," + //
+			"        accumulate: '" + ACC_FUNCTION + "'," + //
+			"        accumulateArgs: [\"$copies\"]," + //
+			"        merge: '" + MERGE_FUNCTION + "'," + //
+			"        finalize: '" + FINALIZE_FUNCTION + "'," + //
+			"        lang: \"js\"" + //
+			"      }" + //
+			"    }" + //
+			"  }");
+
+	@Test // DATAMONGO-2623
+	void functionWithoutArgsShouldBeRenderedCorrectly() {
+
+		assertThat(function(FUNCTION_BODY).toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo($function(EMPTY_ARGS_FUNCTION_DOCUMENT));
+	}
+
+	@Test // DATAMONGO-2623
+	void functionWithArgsShouldBeRenderedCorrectly() {
+
+		assertThat(function(FUNCTION_BODY).args("$name").toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo(
+				$function(new Document(EMPTY_ARGS_FUNCTION_DOCUMENT).append("args", Collections.singletonList("$name"))));
+	}
+
+	@Test // DATAMONGO-2623
+	void accumulatorWithStringInput() {
+
+		Accumulator accumulator = accumulatorBuilder() //
+				.init(INIT_FUNCTION) //
+				.accumulate(ACC_FUNCTION).accumulateArgs("$copies") //
+				.merge(MERGE_FUNCTION) //
+				.finalize(FINALIZE_FUNCTION);
+
+		assertThat(accumulator.toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo($ACCUMULATOR);
+	}
+
+	@Test // DATAMONGO-2623
+	void accumulatorWithFunctionInput() {
+
+		Accumulator accumulator = accumulatorBuilder() //
+				.init(function(INIT_FUNCTION)) //
+				.accumulate(function(ACC_FUNCTION).args("$copies")) //
+				.merge(MERGE_FUNCTION) //
+				.finalize(FINALIZE_FUNCTION);
+
+		assertThat(accumulator.toDocument(Aggregation.DEFAULT_CONTEXT)).isEqualTo($ACCUMULATOR);
+	}
+
+	static Document $function(Document source) {
+		return new Document("$function", source);
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/UnionWithOperationUnitTests.java

@@ -0,0 +1,131 @@
+/*
+ * 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 static org.assertj.core.api.Assertions.*;
+
+import java.util.Arrays;
+import java.util.List;
+
+import org.bson.Document;
+import org.junit.jupiter.api.Test;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
+import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
+import org.springframework.data.mongodb.core.convert.QueryMapper;
+import org.springframework.data.mongodb.core.mapping.Field;
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
+import org.springframework.lang.Nullable;
+
+/**
+ * Unit tests for {@link UnionWithOperation}.
+ *
+ * @author Christoph Strobl
+ */
+class UnionWithOperationUnitTests {
+
+	@Test // DATAMONGO-2622
+	void throwsErrorWhenNoCollectionPresent() {
+		assertThatExceptionOfType(IllegalArgumentException.class).isThrownBy(() -> UnionWithOperation.unionWith(null));
+	}
+
+	@Test // DATAMONGO-2622
+	void rendersJustCollectionCorrectly() {
+
+		assertThat(UnionWithOperation.unionWith("coll-1").toPipelineStages(contextFor(Warehouse.class)))
+				.containsExactly(new Document("$unionWith", new Document("coll", "coll-1")));
+	}
+
+	@Test // DATAMONGO-2622
+	void rendersPipelineCorrectly() {
+
+		assertThat(UnionWithOperation.unionWith("coll-1").mapFieldsTo(Warehouse.class)
+				.pipeline(Aggregation.project().and("location").as("region")).toPipelineStages(contextFor(Warehouse.class)))
+						.containsExactly(new Document("$unionWith", new Document("coll", "coll-1").append("pipeline",
+								Arrays.asList(new Document("$project", new Document("region", 1))))));
+	}
+
+	@Test // DATAMONGO-2622
+	void rendersPipelineCorrectlyForDifferentDomainType() {
+
+		assertThat(UnionWithOperation.unionWith("coll-1").pipeline(Aggregation.project().and("name").as("name"))
+				.mapFieldsTo(Supplier.class).toPipelineStages(contextFor(Warehouse.class)))
+						.containsExactly(new Document("$unionWith", new Document("coll", "coll-1").append("pipeline",
+								Arrays.asList(new Document("$project", new Document("name", "$supplier"))))));
+	}
+
+	@Test // DATAMONGO-2622
+	void rendersPipelineCorrectlyForUntypedContext() {
+
+		assertThat(UnionWithOperation.unionWith("coll-1").pipeline(Aggregation.project("region"))
+				.toPipelineStages(contextFor(null)))
+						.containsExactly(new Document("$unionWith", new Document("coll", "coll-1").append("pipeline",
+								Arrays.asList(new Document("$project", new Document("region", 1))))));
+	}
+
+	@Test // DATAMONGO-2622
+	void doesNotMapAgainstFieldsFromAPreviousStage() {
+
+		TypedAggregation<Supplier> agg = TypedAggregation.newAggregation(Supplier.class,
+				Aggregation.project().and("name").as("supplier"),
+				UnionWithOperation.unionWith("coll-1").pipeline(Aggregation.project().and("name").as("name")));
+
+		List<Document> pipeline = agg.toPipeline(contextFor(Supplier.class));
+		System.out.println("pipeline: " + pipeline);
+		assertThat(pipeline).containsExactly(new Document("$project", new Document("supplier", 1)), //
+				new Document("$unionWith", new Document("coll", "coll-1").append("pipeline",
+						Arrays.asList(new Document("$project", new Document("name", 1))))));
+	}
+
+	@Test // DATAMONGO-2622
+	void mapAgainstUnionWithDomainTypeEvenWhenInsideTypedAggregation() {
+
+		TypedAggregation<Supplier> agg = TypedAggregation.newAggregation(Supplier.class,
+				Aggregation.project().and("name").as("supplier"), UnionWithOperation.unionWith("coll-1")
+						.mapFieldsTo(Warehouse.class).pipeline(Aggregation.project().and("location").as("location")));
+
+		List<Document> pipeline = agg.toPipeline(contextFor(Supplier.class));
+		assertThat(pipeline).containsExactly(new Document("$project", new Document("supplier", 1)), //
+				new Document("$unionWith", new Document("coll", "coll-1").append("pipeline",
+						Arrays.asList(new Document("$project", new Document("location", "$region"))))));
+	}
+
+	private static AggregationOperationContext contextFor(@Nullable Class<?> type) {
+
+		if (type == null) {
+			return Aggregation.DEFAULT_CONTEXT;
+		}
+
+		MappingMongoConverter mongoConverter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE,
+				new MongoMappingContext());
+		mongoConverter.afterPropertiesSet();
+
+		return new TypeBasedAggregationOperationContext(type, mongoConverter.getMappingContext(),
+				new QueryMapper(mongoConverter));
+	}
+
+	static class Warehouse {
+
+		String name;
+		@Field("region") String location;
+		String state;
+	}
+
+	static class Supplier {
+
+		@Field("supplier") String name;
+		String state;
+	}
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/convert/MappingMongoConverterUnitTests.java

@@ -23,6 +23,7 @@ import static org.springframework.data.mongodb.core.DocumentTestUtils.*;
 import lombok.EqualsAndHashCode;
 import lombok.RequiredArgsConstructor;
 
+import java.lang.annotation.Annotation;
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.net.URL;
@@ -70,6 +71,7 @@ import org.springframework.data.mongodb.core.convert.DocumentAccessorUnitTests.N
 import org.springframework.data.mongodb.core.convert.DocumentAccessorUnitTests.ProjectingType;
 import org.springframework.data.mongodb.core.convert.MappingMongoConverterUnitTests.ClassWithMapUsingEnumAsKey.FooBarEnum;
 import org.springframework.data.mongodb.core.geo.Sphere;
+import org.springframework.data.mongodb.core.mapping.BasicMongoPersistentEntity;
 import org.springframework.data.mongodb.core.mapping.Document;
 import org.springframework.data.mongodb.core.mapping.Field;
 import org.springframework.data.mongodb.core.mapping.FieldType;
@@ -78,7 +80,9 @@ import org.springframework.data.mongodb.core.mapping.MongoPersistentProperty;
 import org.springframework.data.mongodb.core.mapping.PersonPojoStringId;
 import org.springframework.data.mongodb.core.mapping.TextScore;
 import org.springframework.data.mongodb.core.mapping.event.AfterConvertCallback;
+import org.springframework.data.mongodb.buildtimetypeinfo.AddressTypeInformation;
 import org.springframework.data.util.ClassTypeInformation;
+import org.springframework.data.mongodb.buildtimetypeinfo.PersonTypeInformation;
 import org.springframework.test.util.ReflectionTestUtils;
 
 import com.mongodb.BasicDBList;
@@ -2179,6 +2183,108 @@ public class MappingMongoConverterUnitTests {
 		assertThat(((LinkedHashMap) result.get("cluster")).get("_id")).isEqualTo(100L);
 	}
 
+//	@Test
+//	public void perf1() {
+//
+//		ClassTypeInformation.warmCache(PersonTypeInformation.instance(), AddressTypeInformation.instance());
+//
+//		MongoMappingContext mappingContext = new MongoMappingContext();
+//		mappingContext.setInitialEntitySet(new LinkedHashSet<>(
+//				Arrays.asList(org.springframework.data.mongodb.xxx.Person.class, org.springframework.data.mongodb.xxx.Address.class)));
+//		mappingContext.initialize();
+//
+//		MappingMongoConverter converter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext);
+//
+//		org.springframework.data.mongodb.xxx.Person source = new org.springframework.data.mongodb.xxx.Person("spring", "data");
+//		source.setAddress(new org.springframework.data.mongodb.xxx.Address("the city", "never sleeps"));
+//		source.setAge(10);
+//		source = source.withId(9876);
+//		source.setNicknames(Arrays.asList("tick", "trick", "track"));
+//
+//		StopWatch stopWatch = new StopWatch();
+//
+//		List<org.bson.Document> sources = new ArrayList<>();
+//		stopWatch.start("write");
+//		for (int i = 0; i < 10000; i++) {
+//
+//			org.bson.Document targetDocument = new org.bson.Document();
+//			converter.write(source, targetDocument);
+//
+//			sources.add(targetDocument);
+//		}
+//		stopWatch.stop();
+//
+//		stopWatch.start("read");
+//		for (org.bson.Document sourceDoc : sources) {
+//			assertThat(converter.read(org.springframework.data.mongodb.xxx.Person.class, sourceDoc)).isEqualTo(source);
+//		}
+//		stopWatch.stop();
+//
+//		System.out.println(stopWatch.prettyPrint());
+//
+//	}
+
+	// public void perf2() {
+	//
+	// ClassTypeInformation.warmCache(new PersonTypeInformation(), new AddressTypeInformation());
+	//
+	// MongoMappingContext mappingContext = new MongoMappingContext();
+	// mappingContext.setInitialEntitySet(new LinkedHashSet<>(Arrays.asList(org.springframework.data.mongodb.xxx.Person.class,
+	// org.springframework.data.mongodb.xxx.Address.class)));
+	// mappingContext.initialize();
+	//
+	// MappingMongoConverter converter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext);
+	//
+	// org.springframework.data.mongodb.xxx.Person source = new org.springframework.data.mongodb.xxx.Person("spring", "data");
+	// source.setAddress(new org.springframework.data.mongodb.xxx.Address("the city", "never sleeps"));
+	// source.setAge(10);
+	// source.setId(9876);
+	// source.setNicknames(Arrays.asList("tick", "trick", "track"));
+	//
+	// }
+
+	@Test
+	public void staticEntityMetadata() {
+
+		ClassTypeInformation.warmCache(PersonTypeInformation.instance(), AddressTypeInformation.instance());
+
+		MongoMappingContext mappingContext = new MongoMappingContext();
+		mappingContext.setInitialEntitySet(new LinkedHashSet<>(
+				Arrays.asList(org.springframework.data.mongodb.buildtimetypeinfo.Person.class, org.springframework.data.mongodb.buildtimetypeinfo.Address.class)));
+		mappingContext.initialize();
+
+		org.springframework.data.mongodb.buildtimetypeinfo.Person source = new org.springframework.data.mongodb.buildtimetypeinfo.Person("spring", "data");
+		source.setAddress(new org.springframework.data.mongodb.buildtimetypeinfo.Address("the city", "never sleeps"));
+		source.setAge(10);
+		source = source.withId(9876);
+		source.setNicknames(Arrays.asList("tick", "trick", "track"));
+
+		MappingMongoConverter converter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext);
+		org.bson.Document targetDocument = new org.bson.Document();
+		System.out.println();
+		System.out.println("------ WRITE -------");
+		converter.write(source, targetDocument);
+		System.out.println();
+
+		System.out.println("targetDocument: " + targetDocument);
+		System.out.println();
+		System.out.println("------ READ -------");
+		assertThat(targetDocument).containsEntry("_id", 9876L);
+		assertThat(targetDocument).containsEntry("first-name", "spring");
+		assertThat(targetDocument).containsEntry("address",
+				new org.bson.Document("city", "the city").append("street", "never sleeps"));
+		assertThat(targetDocument).containsEntry("nicknames", Arrays.asList("tick", "trick", "track"));
+
+		org.springframework.data.mongodb.buildtimetypeinfo.Person targetEntity = converter.read(org.springframework.data.mongodb.buildtimetypeinfo.Person.class,
+				targetDocument);
+		System.out.println();
+		System.out.println("targetEntity: " + targetEntity);
+		assertThat(targetEntity).isEqualTo(source);
+
+		BasicMongoPersistentEntity<?> entity = mappingContext.getPersistentEntity(org.springframework.data.mongodb.buildtimetypeinfo.Person.class);
+		assertThat(entity.getCollection()).isEqualTo("star-wars");
+	}
+
 	static class GenericType<T> {
 		T content;
 	}
@@ -2649,4 +2755,34 @@ public class MappingMongoConverterUnitTests {
 			return entity;
 		}
 	}
+
+	void xxx2() {
+
+		new Field() {
+			@Override
+			public Class<? extends Annotation> annotationType() {
+				return null;
+			}
+
+			@Override
+			public String value() {
+				return null;
+			}
+
+			@Override
+			public String name() {
+				return null;
+			}
+
+			@Override
+			public int order() {
+				return 0;
+			}
+
+			@Override
+			public FieldType targetType() {
+				return null;
+			}
+		};
+	}
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/query/FieldUnitTests.java

@@ -15,29 +15,40 @@
  */
 package org.springframework.data.mongodb.core.query;
 
-import static org.assertj.core.api.Assertions.*;
+import static org.springframework.data.mongodb.test.util.Assertions.*;
 
 import org.junit.jupiter.api.Test;
 
 /**
- * Unit tests for {@link DocumentField}.
+ * Unit tests for {@link Field}.
  *
  * @author Oliver Gierke
+ * @author Owen Q
+ * @author Mark Paluch
  */
-public class FieldUnitTests {
+class FieldUnitTests {
 
 	@Test
-	public void sameObjectSetupCreatesEqualField() {
+	void sameObjectSetupCreatesEqualField() {
 
 		Field left = new Field().elemMatch("key", Criteria.where("foo").is("bar"));
 		Field right = new Field().elemMatch("key", Criteria.where("foo").is("bar"));
 
 		assertThat(left).isEqualTo(right);
 		assertThat(right).isEqualTo(left);
+		assertThat(left.getFieldsObject()).isEqualTo("{key: { $elemMatch: {foo:\"bar\"}}}");
+	}
+
+	@Test // DATAMONGO-2294
+	void rendersInclusionCorrectly() {
+
+		Field fields = new Field().include("foo", "bar").include("baz");
+
+		assertThat(fields.getFieldsObject()).isEqualTo("{foo:1, bar:1, baz:1}");
 	}
 
 	@Test
-	public void differentObjectSetupCreatesEqualField() {
+	void differentObjectSetupCreatesEqualField() {
 
 		Field left = new Field().elemMatch("key", Criteria.where("foo").is("bar"));
 		Field right = new Field().elemMatch("key", Criteria.where("foo").is("foo"));
@@ -45,4 +56,12 @@ public class FieldUnitTests {
 		assertThat(left).isNotEqualTo(right);
 		assertThat(right).isNotEqualTo(left);
 	}
+
+	@Test // DATAMONGO-2294
+	void rendersExclusionCorrectly() {
+
+		Field fields = new Field().exclude("foo", "bar").exclude("baz");
+
+		assertThat(fields.getFieldsObject()).isEqualTo("{foo:0, bar:0, baz:0}");
+	}
 }

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/staticmetadata/StaticMetadataTests.java

@@ -0,0 +1,110 @@
+/*
+ * 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
+ *
+ *      http://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.
+ */
+
+/*
+ * 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
+ *
+ *      http://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.staticmetadata;
+
+import static org.assertj.core.api.Assertions.*;
+import static org.springframework.data.mongodb.core.query.Criteria.*;
+import static org.springframework.data.mongodb.core.query.Query.*;
+
+import java.util.Arrays;
+import java.util.LinkedHashSet;
+
+import org.bson.Document;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.data.mongodb.core.MongoTemplate;
+import org.springframework.data.mongodb.core.SimpleMongoClientDatabaseFactory;
+import org.springframework.data.mongodb.core.convert.MappingMongoConverter;
+import org.springframework.data.mongodb.core.convert.NoOpDbRefResolver;
+import org.springframework.data.mongodb.core.mapping.MongoMappingContext;
+import org.springframework.data.mongodb.buildtimetypeinfo.Address;
+import org.springframework.data.mongodb.buildtimetypeinfo.AddressTypeInformation;
+import org.springframework.data.util.ClassTypeInformation;
+import org.springframework.data.mongodb.buildtimetypeinfo.Person;
+import org.springframework.data.mongodb.buildtimetypeinfo.PersonTypeInformation;
+
+import com.mongodb.client.MongoClients;
+
+/**
+ * @author Christoph Strobl
+ * @since 2020/10
+ */
+public class StaticMetadataTests {
+
+	MongoMappingContext mappingContext;
+	MappingMongoConverter mongoConverter;
+	MongoTemplate template;
+
+	Person luke;
+
+	@BeforeAll
+	static void beforeAll() {
+		ClassTypeInformation.warmCache(PersonTypeInformation.instance(), AddressTypeInformation.instance());
+	}
+
+	@BeforeEach
+	void beforeEach() {
+
+		mappingContext = new MongoMappingContext();
+		mappingContext.setInitialEntitySet(new LinkedHashSet<>(
+				Arrays.asList(Person.class, Address.class)));
+		mappingContext.initialize();
+
+		mongoConverter = new MappingMongoConverter(NoOpDbRefResolver.INSTANCE, mappingContext);
+		mongoConverter.afterPropertiesSet();
+
+		template = new MongoTemplate(new SimpleMongoClientDatabaseFactory(MongoClients.create(), "sem"), mongoConverter);
+
+		luke = new Person("luke", "skywalker");
+		luke.setAddress(new Address("Mos Eisley", "WB154"));
+		luke.setAge(22);
+		luke = luke.withId(9876);
+		luke.setNicknames(Arrays.asList("jedi", "wormie"));
+	}
+
+	@Test
+	void readWrite() {
+
+		template.save(luke);
+
+		Document savedDocument = template.execute("star-wars",
+				collection -> collection.find(new Document("_id", luke.getId())).first());
+		System.out.println("savedDocument.toJson(): " + savedDocument.toJson());
+
+		Person savedEntity = template.findOne(query(where("id").is(luke.getId())), Person.class);
+		System.out.println("savedEntity: " + savedEntity);
+
+		assertThat(savedEntity).isEqualTo(luke);
+	}
+
+}

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/util/json/ParameterBindingJsonReaderUnitTests.java

@@ -374,6 +374,15 @@ class ParameterBindingJsonReaderUnitTests {
 				.isEqualTo(Document.parse("{ $and: [{'fieldA': {$in: [/ABC.*/, /CDE.*F/]}}, {'fieldB': {$ne: null}}]}"));
 	}
 
+	@Test // DATAMONGO-2633
+	void shouldParseNestedArrays() {
+
+		Document target = parse("{ 'stores.location' : { $geoWithin: { $centerSphere: [ [ ?0, 48.799029 ] , ?1 ] } } }",
+				1.948516D, 0.004D);
+		assertThat(target).isEqualTo(Document
+				.parse("{ 'stores.location' : { $geoWithin: { $centerSphere: [ [ 1.948516, 48.799029 ] , 0.004 ] } } }"));
+	}
+
 	private static Document parse(String json, Object... args) {
 
 		ParameterBindingJsonReader reader = new ParameterBindingJsonReader(json, args);

spring-data-mongodb/src/test/kotlin/org/springframework/data/mongodb/core/query/KPropertyPathTests.kt

@@ -19,7 +19,11 @@ import org.assertj.core.api.Assertions.assertThat
 import org.junit.Test
 
 /**
+ * Unit tests for [KPropertyPath] and its extensions.
+ *
  * @author Tjeu Kayim
+ * @author Yoann de Martino
+ * @author Mark Paluch
  */
 class KPropertyPathTests {
 
@@ -60,6 +64,35 @@ class KPropertyPathTests {
 		assertThat(property).isEqualTo("entity.book.author.name")
 	}
 
+	@Test
+	fun `Convert simple KProperty to property path using toPath`() {
+
+		class AnotherEntity(val entity: String)
+
+		val property = (AnotherEntity::entity).toPath()
+
+		assertThat(property).isEqualTo("entity")
+	}
+
+	@Test
+	fun `Convert nested KProperty to field name using toPath()`() {
+
+		val property = (Book::author / Author::name).toPath()
+
+		assertThat(property).isEqualTo("author.name")
+	}
+
+	@Test
+	fun `Convert triple nested KProperty to property path using toPath`() {
+
+		class Entity(val book: Book)
+		class AnotherEntity(val entity: Entity)
+
+		val property = (AnotherEntity::entity / Entity::book / Book::author / Author::name).toPath()
+
+		assertThat(property).isEqualTo("entity.book.author.name")
+	}
+
 	class Book(val title: String, val author: Author)
 	class Author(val name: String)
 }

spring-data-mongodb/src/test/kotlin/org/springframework/data/mongodb/repository/query/ReactiveMongoQueryMethodCoroutineUnitTests.kt

@@ -39,6 +39,8 @@ class ReactiveMongoQueryMethodCoroutineUnitTests {
 		suspend fun findSuspendAllByName(): Flow<Person>
 
 		fun findAllByName(): Flow<Person>
+
+		suspend fun findSuspendByName(): List<Person>
 	}
 
 	@Test // DATAMONGO-2562
@@ -58,4 +60,13 @@ class ReactiveMongoQueryMethodCoroutineUnitTests {
 
 		assertThat(queryMethod.isCollectionQuery).isTrue()
 	}
+
+	@Test // DATAMONGO-2630
+	internal fun `should consider suspended methods returning List as collection queries`() {
+
+		val method = PersonRepository::class.java.getMethod("findSuspendByName", Continuation::class.java)
+		val queryMethod = ReactiveMongoQueryMethod(method, DefaultRepositoryMetadata(PersonRepository::class.java), projectionFactory, MongoMappingContext())
+
+		assertThat(queryMethod.isCollectionQuery).isTrue()
+	}
 }

src/main/asciidoc/new-features.adoc

@@ -7,6 +7,8 @@
 * <<mongo.auditing,Reactive auditing>> enabled through `@EnableReactiveMongoAuditing`. `@EnableMongoAuditing` no longer registers `ReactiveAuditingEntityCallback`.
 * Reactive SpEL support in `@Query` and `@Aggregation` query methods.
 * Aggregation hints via `AggregationOptions.builder().hint(bson).build()`.
+* Extension Function `KProperty.asPath()` to render property references into a property path representation.
+* Server-side JavaScript aggregation expressions `$function` and `$accumulator` via `ScriptOperators`.
 
 [[new-features.3.0]]
 == What's New in Spring Data MongoDB 3.0

src/main/asciidoc/reference/mongodb.adoc

@@ -2559,6 +2559,7 @@ The MongoDB Aggregation Framework provides the following types of aggregation op
 * Lookup Aggregation Operators
 * Convert Aggregation Operators
 * Object Aggregation Operators
+* Script Aggregation Operators
 
 At the time of this writing, we provide support for the following Aggregation Operations in Spring Data MongoDB:
 
@@ -2606,6 +2607,9 @@ At the time of this writing, we provide support for the following Aggregation Op
 
 | Object Aggregation Operators
 | `objectToArray`, `mergeObjects`
+
+| Script Aggregation Operators
+| `function`, `accumulator`
 |===
 
 * The operation is mapped or added by Spring Data MongoDB.