DATAMONGO-1550 - Add $replaceRoot aggregation stage.

We now support the $replaceRoot stage in aggregation pipelines. $replaceRoot can reference either a field, an aggregation expression or it can be used to compose a replacement document.

newAggregation(
	replaceRoot().withDocument()
		.andValue("value").as("field")
		.and(MULTIPLY.of(field("total"), field("discounted")))
);

newAggregation(
	replaceRoot("item")));

Original Pull Request: spring-projects#422

Author: mp911de

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

@@ -29,6 +29,8 @@
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.ExposedField;
 import org.springframework.data.mongodb.core.aggregation.ExposedFields.FieldReference;
 import org.springframework.data.mongodb.core.aggregation.FieldsExposingAggregationOperation.InheritsFieldsAggregationOperation;
+import org.springframework.data.mongodb.core.aggregation.ReplaceRootOperation.ReplaceRootDocumentOperationBuilder;
+import org.springframework.data.mongodb.core.aggregation.ReplaceRootOperation.ReplaceRootOperationBuilder;
 import org.springframework.data.mongodb.core.aggregation.Fields.*;
 import org.springframework.data.mongodb.core.query.Criteria;
 import org.springframework.data.mongodb.core.query.CriteriaDefinition;
@@ -216,6 +218,40 @@ public static UnwindOperation unwind(String field) {
 		return new UnwindOperation(field(field));
 	}
 
+	/**
+	 * Factory method to create a new {@link ReplaceRootOperation} for the field with the given name.
+	 *
+	 * @param fieldName must not be {@literal null} or empty.
+	 * @return
+	 * @since 1.10
+	 */
+	public static ReplaceRootOperation replaceRoot(String fieldName) {
+		return ReplaceRootOperation.builder().withValueOf(fieldName);
+	}
+
+	/**
+	 * Factory method to create a new {@link ReplaceRootOperation} for the field with the given
+	 * {@link AggregationExpression}.
+	 *
+	 * @param aggregationExpression must not be {@literal null}.
+	 * @return
+	 * @since 1.10
+	 */
+	public static ReplaceRootOperation replaceRoot(AggregationExpression aggregationExpression) {
+		return ReplaceRootOperation.builder().withValueOf(aggregationExpression);
+	}
+
+	/**
+	 * Factory method to create a new {@link ReplaceRootDocumentOperationBuilder} to configure a
+	 * {@link ReplaceRootOperation}.
+	 *
+	 * @return the {@literal ReplaceRootDocumentOperationBuilder}.
+	 * @since 1.10
+	 */
+	public static ReplaceRootOperationBuilder replaceRoot() {
+		return ReplaceRootOperation.builder();
+	}
+
 	/**
 	 * Factory method to create a new {@link UnwindOperation} for the field with the given name and
 	 * {@code preserveNullAndEmptyArrays}. Note that extended unwind is supported in MongoDB version 3.2+.
@@ -468,11 +504,13 @@ public Document toDocument(String inputCollectionName, AggregationOperationConte
 			if (operation instanceof FieldsExposingAggregationOperation) {
 
 				FieldsExposingAggregationOperation exposedFieldsOperation = (FieldsExposingAggregationOperation) operation;
+				ExposedFields fields = exposedFieldsOperation.getFields();
 
 				if (operation instanceof InheritsFieldsAggregationOperation) {
-					context = new InheritingExposedFieldsAggregationOperationContext(exposedFieldsOperation.getFields(), context);
+					context = new InheritingExposedFieldsAggregationOperationContext(fields, context);
 				} else {
-					context = new ExposedFieldsAggregationOperationContext(exposedFieldsOperation.getFields(), context);
+					context = fields.exposesNoFields() ? DEFAULT_CONTEXT
+							: new ExposedFieldsAggregationOperationContext(fields, context);
 				}
 			}
 		}