From 3dc6cab13284ddc54e6732513fb5c9d19cf7af48 Mon Sep 17 00:00:00 2001
From: Christoph Strobl <cstrobl@pivotal.io>
Date: Tue, 14 Aug 2018 09:39:50 +0200
Subject: [PATCH] DATAMONGO-2053 - Add support for $mergeObjects aggregation
 operator.

Original pull request: #601.
---
 .../AbstractAggregationExpression.java        |   5 +
 .../core/aggregation/ObjectOperators.java     | 228 ++++++++++++++++++
 .../aggregation/ObjectOperatorsUnitTests.java |  91 +++++++
 src/main/asciidoc/reference/mongodb.adoc      |   4 +
 4 files changed, 328 insertions(+)
 create mode 100644 spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ObjectOperators.java
 create mode 100644 spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ObjectOperatorsUnitTests.java

diff --git a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.java b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.java
index ec0a8ff8f..ecdea2166 100644
--- a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.java
+++ b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/AbstractAggregationExpression.java
@@ -23,6 +23,7 @@ import java.util.List;
 import java.util.Map;
 
 import org.bson.Document;
+import org.springframework.data.mongodb.core.aggregation.Aggregation.SystemVariable;
 import org.springframework.util.Assert;
 import org.springframework.util.ObjectUtils;
 
@@ -92,6 +93,10 @@ abstract class AbstractAggregationExpression implements AggregationExpression {
 			return targetDocument;
 		}
 
+		if(value instanceof SystemVariable) {
+			return value.toString();
+		}
+
 		return value;
 	}
 
diff --git a/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ObjectOperators.java b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ObjectOperators.java
new file mode 100644
index 000000000..954d915cb
--- /dev/null
+++ b/spring-data-mongodb/src/main/java/org/springframework/data/mongodb/core/aggregation/ObjectOperators.java
@@ -0,0 +1,228 @@
+/*
+ * Copyright 2018 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.aggregation;
+
+import java.util.Arrays;
+import java.util.Collection;
+
+import org.bson.Document;
+import org.springframework.lang.Nullable;
+import org.springframework.util.Assert;
+
+/**
+ * Abstraction for
+ * <a href="https://docs.mongodb.com/manual/meta/aggregation-quick-reference/#object-expression-operators">object
+ * expression operators</a>.
+ *
+ * @author Christoph Strobl
+ * @since 2.1
+ */
+public class ObjectOperators {
+
+	/**
+	 * Take the value referenced by given {@literal fieldReference}.
+	 *
+	 * @param fieldReference must not be {@literal null}.
+	 * @return new instance of {@link ObjectOperatorFactory}.
+	 */
+	public static ObjectOperatorFactory valueOf(String fieldReference) {
+		return new ObjectOperatorFactory(Fields.field(fieldReference));
+	}
+
+	/**
+	 * Take the value provided by the given {@link AggregationExpression}.
+	 *
+	 * @param expression must not be {@literal null}.
+	 * @return new instance of {@link ObjectOperatorFactory}.
+	 */
+	public static ObjectOperatorFactory valueOf(AggregationExpression expression) {
+		return new ObjectOperatorFactory(expression);
+	}
+
+	/**
+	 * @author Christoph Strobl
+	 */
+	public static class ObjectOperatorFactory {
+
+		@Nullable private final Object value;
+
+		/**
+		 * Creates new {@link ObjectOperatorFactory} for given {@literal value}.
+		 *
+		 * @param value must not be {@literal null}.
+		 */
+		public ObjectOperatorFactory(Object value) {
+
+			Assert.notNull(value, "Value must not be null!");
+			this.value = value;
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} that takes the associated value and uses
+		 * {@literal $mergeObjects} as an accumulator within the {@literal $group} stage. <br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public MergeObjects merge() {
+			return MergeObjects.merge(value);
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} that takes the associated value and combines it with the
+		 * given values into a single document. <br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public MergeObjects mergeWith(Object... values) {
+			return merge().mergeWith(values);
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} that takes the associated value and combines it with the
+		 * values of the given {@link Field field references} into a single document. <br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public MergeObjects mergeWithValuesOf(String... fieldReferences) {
+			return merge().mergeWithValuesOf(fieldReferences);
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} that takes the associated value and combines it with the
+		 * result values of the given {@link Aggregation expressions} into a single document. <br />
+		 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+		 *
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public MergeObjects mergeWithValuesOf(AggregationExpression... expression) {
+			return merge().mergeWithValuesOf(expression);
+		}
+	}
+
+	/**
+	 * {@link AggregationExpression} for {@code $mergeObjects} that combines multiple documents into a single document.
+	 * <br />
+	 * <strong>NOTE:</strong> Requires MongoDB 4.0 or later.
+	 *
+	 * @author Christoph Strobl
+	 * @see <a href=
+	 *      "https://docs.mongodb.com/manual/reference/operator/aggregation/mergeObjects/">https://docs.mongodb.com/manual/reference/operator/aggregation/mergeObjects/</a>
+	 * @since 2.1
+	 */
+	public static class MergeObjects extends AbstractAggregationExpression {
+
+		private MergeObjects(Object value) {
+			super(value);
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} that takes given values and combines them into a single
+		 * document. <br />
+		 *
+		 * @param values must not be {@literal null}.
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public static MergeObjects merge(Object... values) {
+			return new MergeObjects(Arrays.asList(values));
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} that takes the given {@link Field field references} and
+		 * combines them into a single document.
+		 *
+		 * @param fieldReferences must not be {@literal null}.
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public static MergeObjects mergeValuesOf(String... fieldReferences) {
+			return merge(Arrays.stream(fieldReferences).map(Fields::field).toArray());
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} that takes the result of the given {@link Aggregation
+		 * expressions} and combines them into a single document.
+		 *
+		 * @param expressions must not be {@literal null}.
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public static MergeObjects mergeValuesOf(AggregationExpression... expressions) {
+			return merge(expressions);
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} by adding the given {@link Field field references}.
+		 *
+		 * @param fieldReferences must not be {@literal null}.
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public MergeObjects mergeWithValuesOf(String... fieldReferences) {
+			return mergeWith(Arrays.stream(fieldReferences).map(Fields::field).toArray());
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} by adding the given {@link AggregationExpression
+		 * expressions}.
+		 *
+		 * @param expression must not be {@literal null}.
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public MergeObjects mergeWithValuesOf(AggregationExpression... expression) {
+			return mergeWith(expression);
+		}
+
+		/**
+		 * Creates new {@link MergeObjects aggregation expression} by adding the given values.
+		 *
+		 * @param values must not be {@literal null}.
+		 * @return new instance of {@link MergeObjects}.
+		 */
+		public MergeObjects mergeWith(Object... values) {
+			return new MergeObjects(append(Arrays.asList(values)));
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AbstractAggregationExpression#toDocument(java.lang.Object, org.springframework.data.mongodb.core.aggregation.AggregationOperationContext)
+		 */
+		@Override
+		public Document toDocument(Object value, AggregationOperationContext context) {
+			return super.toDocument(potentiallyExtractSingleValue(value), context);
+		}
+
+		private Object potentiallyExtractSingleValue(Object value) {
+
+			if (value instanceof Collection) {
+
+				Collection<Object> collection = ((Collection) value);
+				if (collection.size() == 1) {
+					return collection.iterator().next();
+				}
+			}
+			return value;
+		}
+
+		/*
+		 * (non-Javadoc)
+		 * @see org.springframework.data.mongodb.core.aggregation.AbstractAggregationExpression#getMongoMethod()
+		 */
+		@Override
+		protected String getMongoMethod() {
+			return "$mergeObjects";
+		}
+	}
+}
diff --git a/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ObjectOperatorsUnitTests.java b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ObjectOperatorsUnitTests.java
new file mode 100644
index 000000000..c1d6d3484
--- /dev/null
+++ b/spring-data-mongodb/src/test/java/org/springframework/data/mongodb/core/aggregation/ObjectOperatorsUnitTests.java
@@ -0,0 +1,91 @@
+/*
+ * Copyright 2018 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.aggregation;
+
+import static org.assertj.core.api.Assertions.*;
+
+import org.bson.Document;
+import org.junit.Test;
+import org.springframework.data.mongodb.core.aggregation.Aggregation.SystemVariable;
+import org.springframework.data.mongodb.core.aggregation.ObjectOperators.MergeObjects;
+
+/**
+ * Unit tests for {@link ObjectOperators}.
+ *
+ * @author Christoph Strobl
+ * @currentRead Royal Assassin - Robin Hobb
+ */
+public class ObjectOperatorsUnitTests {
+
+	static final String EXPRESSION_STRING = "{ \"$king-in-waiting\" : \"verity\" }";
+	static final Document EXPRESSION_DOC = Document.parse(EXPRESSION_STRING);
+	static final AggregationExpression EXPRESSION = context -> EXPRESSION_DOC;
+
+	@Test // DATAMONGO-2053
+	public void mergeSingleFieldReference() {
+
+		assertThat(ObjectOperators.valueOf("kettricken").merge().toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $mergeObjects: \"$kettricken\" } "));
+	}
+
+	@Test // DATAMONGO-2053
+	public void mergeSingleExpression() {
+
+		assertThat(ObjectOperators.valueOf(EXPRESSION).merge().toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $mergeObjects: " + EXPRESSION_STRING + " } "));
+	}
+
+	@Test // DATAMONGO-2053
+	public void mergeEmpty() {
+
+		assertThat(MergeObjects.merge().toDocument(Aggregation.DEFAULT_CONTEXT))
+				.isEqualTo(Document.parse("{ $mergeObjects: [] } "));
+	}
+
+	@Test // DATAMONGO-2053
+	public void mergeMuliFieldReference() {
+
+		assertThat(
+				ObjectOperators.valueOf("kettricken").mergeWithValuesOf("verity").toDocument(Aggregation.DEFAULT_CONTEXT))
+						.isEqualTo(Document.parse("{ $mergeObjects: [ \"$kettricken\", \"$verity\" ] } "));
+	}
+
+	@Test // DATAMONGO-2053
+	public void mergeMixed() {
+
+		assertThat(
+				ObjectOperators.valueOf("kettricken").mergeWithValuesOf(EXPRESSION).toDocument(Aggregation.DEFAULT_CONTEXT))
+						.isEqualTo(Document.parse("{ $mergeObjects: [ \"$kettricken\", " + EXPRESSION_STRING + " ] } "));
+	}
+
+	@Test // DATAMONGO-2053
+	public void mergeWithSystemVariable() {
+
+		assertThat(
+				ObjectOperators.valueOf(EXPRESSION).mergeWith(SystemVariable.ROOT).toDocument(Aggregation.DEFAULT_CONTEXT))
+						.isEqualTo(Document.parse("{ $mergeObjects: [ " + EXPRESSION_STRING + ", \"$$ROOT\" ] } "));
+	}
+
+	@Test // DATAMONGO-2053
+	public void mergeMany() {
+
+		assertThat(ObjectOperators.valueOf("kettricken").mergeWithValuesOf(EXPRESSION)
+				.mergeWith(new Document("fitz", "chivalry")).toDocument(Aggregation.DEFAULT_CONTEXT))
+						.isEqualTo(Document.parse(
+								"{ $mergeObjects: [ \"$kettricken\", " + EXPRESSION_STRING + ", { \"fitz\" : \"chivalry\" } ] } "));
+	}
+
+}
diff --git a/src/main/asciidoc/reference/mongodb.adoc b/src/main/asciidoc/reference/mongodb.adoc
index ebcf2314f..4e69bf26a 100644
--- a/src/main/asciidoc/reference/mongodb.adoc
+++ b/src/main/asciidoc/reference/mongodb.adoc
@@ -2100,6 +2100,7 @@ The MongoDB Aggregation Framework provides the following types of aggregation op
 * Conditional Aggregation Operators
 * Lookup Aggregation Operators
 * Convert Aggregation Operators
+* Object Aggregation Operators
 
 At the time of this writing, we provide support for the following Aggregation Operations in Spring Data MongoDB:
 
@@ -2144,6 +2145,9 @@ At the time of this writing, we provide support for the following Aggregation Op
 
 | Convert Aggregation Operators
 | `convert`, `toBool`, `toDate`, `toDecimal`, `toDouble`, `toInt`, `toLong`, `toObjectId`, `toString`
+
+| Object Aggregation Operators
+| `mergeObjects`
 |===
 
 * The operation is mapped or added by Spring Data MongoDB.