Skip output for `void` methods using declarative Aggregations having `$out` stage.

We now set the skipOutput flag if an annotated Aggregation defines an $out stage and when the method is declared to return no result (void / Mono<Void>, kotlin.Unit)

Closes: #4088
Original pull request: #4341

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

@@ -127,6 +127,7 @@ public abstract class AbstractMongoQuery implements RepositoryQuery {
 	 * @param accessor for providing invocation arguments. Never {@literal null}.
 	 * @param typeToRead the desired component target type. Can be {@literal null}.
 	 */
+	@Nullable
 	protected Object doExecute(MongoQueryMethod method, ResultProcessor processor, ConvertingParameterAccessor accessor,
 			@Nullable Class<?> typeToRead) {
 

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

@@ -27,6 +27,7 @@ import org.springframework.data.domain.Sort.Order;
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions;
+import org.springframework.data.mongodb.core.aggregation.AggregationPipeline;
 import org.springframework.data.mongodb.core.convert.MongoConverter;
 import org.springframework.data.mongodb.core.query.Collation;
 import org.springframework.data.mongodb.core.query.Meta;
@@ -125,7 +126,7 @@ abstract class AggregationUtils {
 	 * @param accessor
 	 * @param targetType
 	 */
-	static void appendSortIfPresent(List<AggregationOperation> aggregationPipeline, ConvertingParameterAccessor accessor,
+	static void appendSortIfPresent(AggregationPipeline aggregationPipeline, ConvertingParameterAccessor accessor,
 			Class<?> targetType) {
 
 		if (accessor.getSort().isUnsorted()) {
@@ -150,7 +151,7 @@ abstract class AggregationUtils {
 	 * @param aggregationPipeline
 	 * @param accessor
 	 */
-	static void appendLimitAndOffsetIfPresent(List<AggregationOperation> aggregationPipeline,
+	static void appendLimitAndOffsetIfPresent(AggregationPipeline aggregationPipeline,
 			ConvertingParameterAccessor accessor) {
 		appendLimitAndOffsetIfPresent(aggregationPipeline, accessor, LongUnaryOperator.identity(),
 				IntUnaryOperator.identity());
@@ -166,7 +167,7 @@ abstract class AggregationUtils {
 	 * @param limitOperator
 	 * @since 3.3
 	 */
-	static void appendLimitAndOffsetIfPresent(List<AggregationOperation> aggregationPipeline,
+	static void appendLimitAndOffsetIfPresent(AggregationPipeline aggregationPipeline,
 			ConvertingParameterAccessor accessor, LongUnaryOperator offsetOperator, IntUnaryOperator limitOperator) {
 
 		Pageable pageable = accessor.getPageable();

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

@@ -38,6 +38,7 @@ import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.mongodb.core.query.UpdateDefinition;
 import org.springframework.data.support.PageableExecutionUtils;
 import org.springframework.data.util.TypeInformation;
+import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
 
@@ -55,6 +56,7 @@ import com.mongodb.client.result.DeleteResult;
 @FunctionalInterface
 interface MongoQueryExecution {
 
+	@Nullable
 	Object execute(Query query);
 
 	/**

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

@@ -15,6 +15,8 @@
  */
 package org.springframework.data.mongodb.repository.query;
 
+import org.springframework.data.mongodb.core.aggregation.AggregationPipeline;
+import org.springframework.data.util.ReflectionUtils;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
@@ -81,7 +83,7 @@ public class ReactiveStringBasedAggregation extends AbstractReactiveMongoQuery {
 			Class<?> sourceType = method.getDomainClass();
 			Class<?> targetType = typeToRead;
 
-			List<AggregationOperation> pipeline = it;
+			AggregationPipeline pipeline = new AggregationPipeline(it);
 
 			AggregationUtils.appendSortIfPresent(pipeline, accessor, typeToRead);
 			AggregationUtils.appendLimitAndOffsetIfPresent(pipeline, accessor);
@@ -93,10 +95,13 @@ public class ReactiveStringBasedAggregation extends AbstractReactiveMongoQuery {
 				targetType = Document.class;
 			}
 
-			AggregationOptions options = computeOptions(method, accessor);
-			TypedAggregation<?> aggregation = new TypedAggregation<>(sourceType, pipeline, options);
+			AggregationOptions options = computeOptions(method, accessor, pipeline);
+			TypedAggregation<?> aggregation = new TypedAggregation<>(sourceType, pipeline.getOperations(), options);
 
 			Flux<?> flux = reactiveMongoOperations.aggregate(aggregation, targetType);
+			if(ReflectionUtils.isVoid(typeToRead)) {
+				return flux.then();
+			}
 
 			if (isSimpleReturnType && !isRawReturnType) {
 				flux = flux.handle((item, sink) -> {
@@ -121,7 +126,7 @@ public class ReactiveStringBasedAggregation extends AbstractReactiveMongoQuery {
 		return parseAggregationPipeline(getQueryMethod().getAnnotatedAggregation(), accessor);
 	}
 
-	private AggregationOptions computeOptions(MongoQueryMethod method, ConvertingParameterAccessor accessor) {
+	private AggregationOptions computeOptions(MongoQueryMethod method, ConvertingParameterAccessor accessor, AggregationPipeline pipeline) {
 
 		AggregationOptions.Builder builder = Aggregation.newAggregationOptions();
 
@@ -129,6 +134,9 @@ public class ReactiveStringBasedAggregation extends AbstractReactiveMongoQuery {
 				expressionParser, evaluationContextProvider);
 		AggregationUtils.applyMeta(builder, method);
 		AggregationUtils.applyHint(builder, method);
+		if(ReflectionUtils.isVoid(method.getReturnType().getComponentType().getType()) && pipeline.isOutOrMerge()) {
+			builder.skipOutput();
+		}
 
 		return builder.build();
 	}

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

@@ -29,6 +29,7 @@ import org.springframework.data.mongodb.core.MongoOperations;
 import org.springframework.data.mongodb.core.aggregation.Aggregation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOperation;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions;
+import org.springframework.data.mongodb.core.aggregation.AggregationPipeline;
 import org.springframework.data.mongodb.core.aggregation.AggregationOptions.Builder;
 import org.springframework.data.mongodb.core.aggregation.AggregationResults;
 import org.springframework.data.mongodb.core.aggregation.TypedAggregation;
@@ -37,8 +38,12 @@ import org.springframework.data.mongodb.core.mapping.MongoSimpleTypes;
 import org.springframework.data.mongodb.core.query.Query;
 import org.springframework.data.repository.query.QueryMethodEvaluationContextProvider;
 import org.springframework.data.repository.query.ResultProcessor;
+import org.springframework.data.util.ReflectionUtils;
 import org.springframework.expression.ExpressionParser;
+import org.springframework.lang.Nullable;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.CollectionUtils;
+import org.springframework.util.ObjectUtils;
 
 /**
  * {@link AbstractMongoQuery} implementation to run string-based aggregations using
@@ -85,13 +90,14 @@ public class StringBasedAggregation extends AbstractMongoQuery {
 	 * @see org.springframework.data.mongodb.repository.query.AbstractReactiveMongoQuery#doExecute(org.springframework.data.mongodb.repository.query.MongoQueryMethod, org.springframework.data.repository.query.ResultProcessor, org.springframework.data.mongodb.repository.query.ConvertingParameterAccessor, java.lang.Class)
 	 */
 	@Override
+	@Nullable
 	protected Object doExecute(MongoQueryMethod method, ResultProcessor resultProcessor,
 			ConvertingParameterAccessor accessor, Class<?> typeToRead) {
 
 		Class<?> sourceType = method.getDomainClass();
 		Class<?> targetType = typeToRead;
 
-		List<AggregationOperation> pipeline = computePipeline(method, accessor);
+		AggregationPipeline pipeline = computePipeline(method, accessor);
 		AggregationUtils.appendSortIfPresent(pipeline, accessor, typeToRead);
 
 		if (method.isSliceQuery()) {
@@ -112,8 +118,8 @@ public class StringBasedAggregation extends AbstractMongoQuery {
 			targetType = method.getReturnType().getRequiredActualType().getRequiredComponentType().getType();
 		}
 
-		AggregationOptions options = computeOptions(method, accessor);
-		TypedAggregation<?> aggregation = new TypedAggregation<>(sourceType, pipeline, options);
+		AggregationOptions options = computeOptions(method, accessor, pipeline);
+		TypedAggregation<?> aggregation = new TypedAggregation<>(sourceType, pipeline.getOperations(), options);
 
 		if (method.isStreamQuery()) {
 
@@ -127,6 +133,9 @@ public class StringBasedAggregation extends AbstractMongoQuery {
 		}
 
 		AggregationResults<Object> result = (AggregationResults<Object>) mongoOperations.aggregate(aggregation, targetType);
+		if(ReflectionUtils.isVoid(typeToRead)) {
+			return null;
+		}
 
 		if (isRawAggregationResult) {
 			return result;
@@ -168,11 +177,11 @@ public class StringBasedAggregation extends AbstractMongoQuery {
 		return MongoSimpleTypes.HOLDER.isSimpleType(targetType);
 	}
 
-	List<AggregationOperation> computePipeline(MongoQueryMethod method, ConvertingParameterAccessor accessor) {
-		return parseAggregationPipeline(method.getAnnotatedAggregation(), accessor);
+	AggregationPipeline computePipeline(MongoQueryMethod method, ConvertingParameterAccessor accessor) {
+		return new AggregationPipeline(parseAggregationPipeline(method.getAnnotatedAggregation(), accessor));
 	}
 
-	private AggregationOptions computeOptions(MongoQueryMethod method, ConvertingParameterAccessor accessor) {
+	private AggregationOptions computeOptions(MongoQueryMethod method, ConvertingParameterAccessor accessor, AggregationPipeline pipeline) {
 
 		AggregationOptions.Builder builder = Aggregation.newAggregationOptions();
 
@@ -181,6 +190,10 @@ public class StringBasedAggregation extends AbstractMongoQuery {
 		AggregationUtils.applyMeta(builder, method);
 		AggregationUtils.applyHint(builder, method);
 
+		if(ReflectionUtils.isVoid(method.getReturnType().getType()) && pipeline.isOutOrMerge()) {
+			builder.skipOutput();
+		}
+
 		return builder.build();
 	}
 

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/query/ReactiveStringBasedAggregationUnitTests.java

@@ -79,6 +79,7 @@ public class ReactiveStringBasedAggregationUnitTests {
 
 	private static final String RAW_SORT_STRING = "{ '$sort' : { 'lastname' : -1 } }";
 	private static final String RAW_GROUP_BY_LASTNAME_STRING = "{ '$group': { '_id' : '$lastname', 'names' : { '$addToSet' : '$firstname' } } }";
+	private static final String RAW_OUT = "{ '$out' : 'authors' }";
 	private static final String GROUP_BY_LASTNAME_STRING_WITH_PARAMETER_PLACEHOLDER = "{ '$group': { '_id' : '$lastname', names : { '$addToSet' : '$?0' } } }";
 	private static final String GROUP_BY_LASTNAME_STRING_WITH_SPEL_PARAMETER_PLACEHOLDER = "{ '$group': { '_id' : '$lastname', 'names' : { '$addToSet' : '$?#{[0]}' } } }";
 
@@ -196,6 +197,22 @@ public class ReactiveStringBasedAggregationUnitTests {
 		return new AggregationInvocation(aggregationCaptor.getValue(), targetTypeCaptor.getValue(), result);
 	}
 
+	@Test // GH-4088
+	void aggregateWithVoidReturnTypeSkipsResultOnOutStage() {
+
+		AggregationInvocation invocation = executeAggregation("outSkipResult");
+
+		assertThat(skipResultsOf(invocation)).isTrue();
+	}
+
+	@Test // GH-4088
+	void aggregateWithOutStageDoesNotSkipResults() {
+
+		AggregationInvocation invocation = executeAggregation("outDoNotSkipResult");
+
+		assertThat(skipResultsOf(invocation)).isFalse();
+	}
+
 	private ReactiveStringBasedAggregation createAggregationForMethod(String name, Class<?>... parameters) {
 
 		Method method = ClassUtils.getMethod(SampleRepository.class, name, parameters);
@@ -230,6 +247,11 @@ public class ReactiveStringBasedAggregationUnitTests {
 				: null;
 	}
 
+	private Boolean skipResultsOf(AggregationInvocation invocation) {
+		return invocation.aggregation.getOptions() != null ? invocation.aggregation.getOptions().isSkipResults()
+				: false;
+	}
+
 	private Class<?> targetTypeOf(AggregationInvocation invocation) {
 		return invocation.getTargetType();
 	}
@@ -261,6 +283,12 @@ public class ReactiveStringBasedAggregationUnitTests {
 		@Hint("idx")
 		@Aggregation(RAW_GROUP_BY_LASTNAME_STRING)
 		String withHint();
+
+		@Aggregation(pipeline = { RAW_GROUP_BY_LASTNAME_STRING, RAW_OUT })
+		Flux<Person> outDoNotSkipResult();
+
+		@Aggregation(pipeline = { RAW_GROUP_BY_LASTNAME_STRING, RAW_OUT })
+		Mono<Void> outSkipResult();
 	}
 
 	static class PersonAggregate {

spring-data-mongodb/src/test/java/org/springframework/data/mongodb/repository/query/StringBasedAggregationUnitTests.java

@@ -92,6 +92,7 @@ public class StringBasedAggregationUnitTests {
 
 	private static final String RAW_SORT_STRING = "{ '$sort' : { 'lastname' : -1 } }";
 	private static final String RAW_GROUP_BY_LASTNAME_STRING = "{ '$group': { '_id' : '$lastname', 'names' : { '$addToSet' : '$firstname' } } }";
+	private static final String RAW_OUT = "{ '$out' : 'authors' }";
 	private static final String GROUP_BY_LASTNAME_STRING_WITH_PARAMETER_PLACEHOLDER = "{ '$group': { '_id' : '$lastname', names : { '$addToSet' : '$?0' } } }";
 	private static final String GROUP_BY_LASTNAME_STRING_WITH_SPEL_PARAMETER_PLACEHOLDER = "{ '$group': { '_id' : '$lastname', 'names' : { '$addToSet' : '$?#{[0]}' } } }";
 
@@ -268,6 +269,22 @@ public class StringBasedAggregationUnitTests {
 		assertThat(hintOf(invocation)).isEqualTo("idx");
 	}
 
+	@Test // GH-4088
+	void aggregateWithVoidReturnTypeSkipsResultOnOutStage() {
+
+		AggregationInvocation invocation = executeAggregation("outSkipResult");
+
+		assertThat(skipResultsOf(invocation)).isTrue();
+	}
+
+	@Test // GH-4088
+	void aggregateWithOutStageDoesNotSkipResults() {
+
+		AggregationInvocation invocation = executeAggregation("outDoNotSkipResult");
+
+		assertThat(skipResultsOf(invocation)).isFalse();
+	}
+
 	private AggregationInvocation executeAggregation(String name, Object... args) {
 
 		Class<?>[] argTypes = Arrays.stream(args).map(Object::getClass).toArray(Class[]::new);
@@ -316,6 +333,11 @@ public class StringBasedAggregationUnitTests {
 				: null;
 	}
 
+	private Boolean skipResultsOf(AggregationInvocation invocation) {
+		return invocation.aggregation.getOptions() != null ? invocation.aggregation.getOptions().isSkipResults()
+				: false;
+	}
+
 	private Class<?> targetTypeOf(AggregationInvocation invocation) {
 		return invocation.getTargetType();
 	}
@@ -368,6 +390,12 @@ public class StringBasedAggregationUnitTests {
 		@Hint("idx")
 		@Aggregation(RAW_GROUP_BY_LASTNAME_STRING)
 		String withHint();
+
+		@Aggregation(pipeline = { RAW_GROUP_BY_LASTNAME_STRING, RAW_OUT })
+		List<Person> outDoNotSkipResult();
+
+		@Aggregation(pipeline = { RAW_GROUP_BY_LASTNAME_STRING, RAW_OUT })
+		void outSkipResult();
 	}
 
 	private interface UnsupportedRepository extends Repository<Person, Long> {

src/main/asciidoc/reference/mongo-repositories-aggregation.adoc

@@ -37,6 +37,12 @@ public interface PersonRepository extends CrudRepository<Person, String> {
 
   @Aggregation("{ '$project': { '_id' : '$lastname' } }")
   List<String> findAllLastnames();                                                 <9>
+
+  @Aggregation(pipeline = {
+		  "{ $group : { _id : '$author', books: { $push: '$title' } } }",
+		  "{ $out : 'authors' }"
+  })
+  void groupAndOutSkippingOutput();                                                <10>
 }
 ----
 [source,java]
@@ -75,6 +81,7 @@ Therefore, the `Sort` properties are mapped against the methods return type `Per
 To gain more control, you might consider `AggregationResult` as method return type as shown in <7>.
 <8> Obtain the raw `AggregationResults` mapped to the generic target wrapper type `SumValue` or `org.bson.Document`.
 <9> Like in <6>, a single value can be directly obtained from multiple result ``Document``s.
+<10> Skips the output of the `$out` stage when return type is `void`.
 ====
 
 In some scenarios, aggregations might require additional options, such as a maximum run time, additional log comments, or the permission to temporarily write data to disk.