Aggregation query method should be able to return Slice and Stream.

Aggregation query methods can not return Slice and Stream.

interface PersonRepository extends CrudReppsitory<Person, String> {

  @Aggregation("{ $group: { _id : $lastname, names : { $addToSet : ?0 } } }")
  Slice<PersonAggregate> groupByLastnameAnd(String property, Pageable page);

  @Aggregation("{ $group: { _id : $lastname, names : { $addToSet : $firstname } } }")
  Stream<PersonAggregate> groupByLastnameAndFirstnamesAsStream();
}

Closes #3543.
Original pull request: #3645.

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

@@ -145,6 +145,28 @@ abstract class AggregationUtils {
 
 		aggregationPipeline.add(Aggregation.limit(pageable.getPageSize()));
 	}
+	
+	/**
+	 * Append {@code $skip} and {@code $limit} aggregation stage if {@link ConvertingParameterAccessor#getSort()} is
+	 * present.
+	 *
+	 * @param aggregationPipeline
+	 * @param accessor
+	 */
+	static void appendModifiedLimitAndOffsetIfPresent(List<AggregationOperation> aggregationPipeline,
+			ConvertingParameterAccessor accessor) {
+
+		Pageable pageable = accessor.getPageable();
+		if (pageable.isUnpaged()) {
+			return;
+		}
+
+		if (pageable.getOffset() > 0) {
+			aggregationPipeline.add(Aggregation.skip(pageable.getOffset()));
+		}
+
+		aggregationPipeline.add(Aggregation.limit(pageable.getPageSize()+1));
+	}
 
 	/**
 	 * Extract a single entry from the given {@link Document}. <br />

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

@@ -20,6 +20,8 @@ import java.util.List;
 import java.util.stream.Collectors;
 
 import org.bson.Document;
+import org.springframework.data.domain.Pageable;
+import org.springframework.data.domain.SliceImpl;
 import org.springframework.data.mapping.model.SpELExpressionEvaluator;
 import org.springframework.data.mongodb.InvalidMongoDbApiUsageException;
 import org.springframework.data.mongodb.core.MongoOperations;
@@ -76,18 +78,17 @@ public class StringBasedAggregation extends AbstractMongoQuery {
 	protected Object doExecute(MongoQueryMethod method, ResultProcessor resultProcessor,
 			ConvertingParameterAccessor accessor, Class<?> typeToRead) {
 
-		if (method.isPageQuery() || method.isSliceQuery()) {
-			throw new InvalidMongoDbApiUsageException(String.format(
-					"Repository aggregation method '%s' does not support '%s' return type. Please use eg. 'List' instead.",
-					method.getName(), method.getReturnType().getType().getSimpleName()));
-		}
-
 		Class<?> sourceType = method.getDomainClass();
 		Class<?> targetType = typeToRead;
 
 		List<AggregationOperation> pipeline = computePipeline(method, accessor);
 		AggregationUtils.appendSortIfPresent(pipeline, accessor, typeToRead);
-		AggregationUtils.appendLimitAndOffsetIfPresent(pipeline, accessor);
+		
+		if (method.isSliceQuery()) {
+			AggregationUtils.appendModifiedLimitAndOffsetIfPresent(pipeline, accessor);
+		}else{
+			AggregationUtils.appendLimitAndOffsetIfPresent(pipeline, accessor);
+		}
 
 		boolean isSimpleReturnType = isSimpleReturnType(typeToRead);
 		boolean isRawAggregationResult = ClassUtils.isAssignable(AggregationResults.class, typeToRead);
@@ -118,7 +119,17 @@ public class StringBasedAggregation extends AbstractMongoQuery {
 
 			return result.getMappedResults();
 		}
-
+		
+		List mappedResults = result.getMappedResults(); 
+		
+		if(method.isSliceQuery()) {
+			
+			Pageable pageable = accessor.getPageable();
+			int pageSize = pageable.getPageSize();
+			boolean hasNext = mappedResults.size() > pageSize;
+			return new SliceImpl<Object>(hasNext ? mappedResults.subList(0, pageSize) : mappedResults, pageable, hasNext);
+		}
+		
 		Object uniqueResult = result.getUniqueMappedResult();
 
 		return isSimpleReturnType

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

@@ -36,6 +36,8 @@ import org.mockito.Mock;
 import org.mockito.junit.jupiter.MockitoExtension;
 import org.mockito.junit.jupiter.MockitoSettings;
 import org.mockito.quality.Strictness;
+import org.springframework.data.domain.Slice;
+import org.springframework.data.domain.SliceImpl;
 import org.springframework.data.domain.Page;
 import org.springframework.data.domain.PageRequest;
 import org.springframework.data.domain.Pageable;
@@ -220,13 +222,10 @@ public class StringBasedAggregationUnitTests {
 	}
 
 	@Test // DATAMONGO-2506
-	public void aggregateRaisesErrorOnInvalidReturnType() {
-
-		StringBasedAggregation sba = createAggregationForMethod("invalidPageReturnType", Pageable.class);
-		assertThatExceptionOfType(InvalidMongoDbApiUsageException.class) //
-				.isThrownBy(() -> sba.execute(new Object[] { PageRequest.of(0, 1) })) //
-				.withMessageContaining("invalidPageReturnType") //
-				.withMessageContaining("Page");
+	public void aggregationWithSliceReturnType() {
+		StringBasedAggregation sba = createAggregationForMethod("aggregationWithSliceReturnType", Pageable.class);
+		Object result = sba.execute(new Object[] { PageRequest.of(0, 1) });
+		assertThat(result.getClass()).isEqualTo(SliceImpl.class);
 	}
 
 	@Test // DATAMONGO-2557
@@ -319,7 +318,7 @@ public class StringBasedAggregationUnitTests {
 		PersonAggregate aggregateWithCollation(Collation collation);
 
 		@Aggregation(RAW_GROUP_BY_LASTNAME_STRING)
-		Page<Person> invalidPageReturnType(Pageable page);
+		Slice<Person> aggregationWithSliceReturnType(Pageable page);
 
 		@Aggregation(RAW_GROUP_BY_LASTNAME_STRING)
 		String simpleReturnType();