GH-2749 - Skip relationship mapping if no relationships are available.

Closes #2749

Author: meistermeier

src/main/java/org/springframework/data/neo4j/core/mapping/DefaultNeo4jEntityConverter.java

@@ -651,20 +651,26 @@ private Optional<Object> createInstanceOfRelationships(Neo4jPersistentProperty p
 		String collectionName = relationshipDescription.generateRelatedNodesCollectionName(genericNodeDescription);
 
 		Value list = values.get(collectionName);
+		boolean relationshipListEmptyOrNull = Values.NULL.equals(list);
 
 		List<Object> relationshipsAndProperties = new ArrayList<>();
 
-		if (Values.NULL.equals(list)) {
+		String elementId = IdentitySupport.getElementId(values);
+		Long internalId = IdentitySupport.getInternalId(values);
+		boolean hasGeneratedIdValue = elementId != null || internalId != null;
+
+		if (relationshipListEmptyOrNull && hasGeneratedIdValue) {
 			String sourceNodeId;
 			Function<Relationship, String> sourceIdSelector;
 			Function<Relationship, String> targetIdSelector = relationshipDescription.isIncoming() ? Relationship::startNodeElementId : Relationship::endNodeElementId;
-			if (IdentitySupport.getElementId(values) == null) {
+
+			if (internalId != null) {
 				// this can happen when someone used dto mapping and added the "classical" approach
-				sourceNodeId = Optional.ofNullable(IdentitySupport.getInternalId(values)).map(l -> Long.toString(l)).orElseThrow();
+				sourceNodeId = Long.toString(internalId);
 				Function<Relationship, Long> hlp = relationshipDescription.isIncoming() ? Relationship::endNodeId : Relationship::startNodeId;
 				sourceIdSelector = hlp.andThen(l -> Long.toString(l));
 			} else {
-				sourceNodeId = IdentitySupport.getElementId(values);
+				sourceNodeId = elementId;
 				sourceIdSelector = relationshipDescription.isIncoming() ? Relationship::endNodeElementId : Relationship::startNodeElementId;
 			}
 
@@ -732,7 +738,7 @@ private Optional<Object> createInstanceOfRelationships(Neo4jPersistentProperty p
 				}
 				allMatchingTypeRelationshipsInResult.removeAll(relationshipsProcessed);
 			}
-		} else {
+		} else if (!relationshipListEmptyOrNull) {
 			for (Value relatedEntity : list.asList(Function.identity())) {
 
 				Neo4jPersistentEntity<?> concreteTargetNodeDescription =

src/test/java/org/springframework/data/neo4j/integration/imperative/RepositoryIT.java

@@ -4439,6 +4439,16 @@ void findByPropertyOnRelationshipWithPropertiesRelatedEntity(
 
 			assertThat(repository.findByHobbiesHobbyName("Bowling").getName()).isEqualTo("Freddie");
 		}
+
+		@Test
+		void findByCustomQueryOnlyWithPropertyReturn(
+				@Autowired PersonWithRelationshipWithPropertiesRepository repository) {
+			doWithSession(session ->
+					session.run(
+							"CREATE (:PersonWithRelationshipWithProperties{name:'Freddie'})-[:LIKES{since: 2020, active: true}]->(:Hobby{name: 'Bowling'})").consume());
+
+			assertThat(repository.justTheNames().getName()).isEqualTo("Freddie");
+		}
 	}
 
 	@Test // GH-2706
@@ -4526,6 +4536,9 @@ interface PersonWithRelationshipWithPropertiesRepository
 		PersonWithRelationshipWithProperties findByHobbiesSinceAndHobbiesActive(int since1, boolean active);
 
 		PersonWithRelationshipWithProperties findByHobbiesHobbyName(String hobbyName);
+
+		@Query("MATCH (p:PersonWithRelationshipWithProperties) return p {.name}")
+		PersonWithRelationshipWithProperties justTheNames();
 	}
 
 	interface PetRepository extends Neo4jRepository<Pet, Long> {

src/test/java/org/springframework/data/neo4j/integration/reactive/ReactiveRepositoryIT.java

@@ -1572,6 +1572,18 @@ void findByPropertyOnRelationshipWithPropertiesOr(
 			StepVerifier.create(repository.findByHobbiesSinceOrHobbiesActive(2019, false)).verifyComplete();
 		}
 
+		@Test
+		void findByCustomQueryOnlyWithPropertyReturn(
+				@Autowired ReactivePersonWithRelationshipWithPropertiesRepository repository) {
+			doWithSession(session ->
+					session.run(
+						"CREATE (:PersonWithRelationshipWithProperties{name:'Freddie'})-[:LIKES{since: 2020, active: true}]->(:Hobby{name: 'Bowling'})").consume()
+			);
+
+			StepVerifier.create(repository.justTheNames())
+					.assertNext(person -> assertThat(person.getName()).isEqualTo("Freddie")).verifyComplete();
+		}
+
 		@Test
 		void findByPropertyOnRelationshipWithPropertiesAnd(
 				@Autowired ReactivePersonWithRelationshipWithPropertiesRepository repository) {
@@ -2744,6 +2756,9 @@ interface ReactivePersonWithRelationshipWithPropertiesRepository
 		Mono<PersonWithRelationshipWithProperties> findByHobbiesSinceOrHobbiesActive(int since1, boolean active);
 
 		Mono<PersonWithRelationshipWithProperties> findByHobbiesSinceAndHobbiesActive(int since1, boolean active);
+
+		@Query("MATCH (p:PersonWithRelationshipWithProperties) return p {.name}")
+		Mono<PersonWithRelationshipWithProperties> justTheNames();
 	}
 
 	interface ReactiveHobbyWithRelationshipWithPropertiesRepository