GH-2305 - Add association support for DTO projections.

Closes #2305

Author: meistermeier

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

@@ -33,6 +33,7 @@
 import org.springframework.data.mapping.PersistentPropertyAccessor;
 import org.springframework.data.mapping.PreferredConstructor;
 import org.springframework.data.mapping.PreferredConstructor.Parameter;
+import org.springframework.data.mapping.PropertyHandler;
 import org.springframework.data.mapping.SimplePropertyHandler;
 import org.springframework.data.mapping.model.ParameterValueProvider;
 import org.springframework.data.util.ClassTypeInformation;
@@ -151,17 +152,22 @@ public Object getParameterValue(Parameter parameter) {
 				});
 
 		PersistentPropertyAccessor<Object> dtoAccessor = targetEntity.getPropertyAccessor(dto);
-		targetEntity.doWithProperties((SimplePropertyHandler) property -> {
+		targetEntity.doWithAll((PropertyHandler) property ->
+				setPropertyOnDtoObject(entityInstanceAndSource, sourceEntity, sourceAccessor, constructor, dtoAccessor, property));
 
-			if (constructor.isConstructorParameter(property)) {
-				return;
-			}
+		return dto;
+	}
 
-			Object propertyValue = getPropertyValueFor(property, sourceEntity, sourceAccessor, entityInstanceAndSource);
-			dtoAccessor.setProperty(property, propertyValue);
-		});
+	private void setPropertyOnDtoObject(EntityInstanceWithSource entityInstanceAndSource, PersistentEntity<?, ?> sourceEntity,
+			PersistentPropertyAccessor<Object> sourceAccessor, PreferredConstructor<?, ?> constructor,
+			PersistentPropertyAccessor<Object> dtoAccessor, PersistentProperty<?> property) {
 
-		return dto;
+		if (constructor.isConstructorParameter(property)) {
+			return;
+		}
+
+		Object propertyValue = getPropertyValueFor(property, sourceEntity, sourceAccessor, entityInstanceAndSource);
+		dtoAccessor.setProperty(property, propertyValue);
 	}
 
 	@Nullable

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

@@ -1288,6 +1288,33 @@ void shouldFindOneToOneWithWildcardReturn(@Autowired OneToOneRepository reposito
 			List<OneToOneSource> oneToOnes = repository.findAllWithCustomQueryReturnStar();
 			assertOneToOneScenario(oneToOnes);
 		}
+
+		private void createOneToOneScenarioForNullValues() {
+			doWithSession(session -> {
+						try (Transaction tx = session.beginTransaction()) {
+							tx.run("CREATE (s:OneToOneSource {name: 's1'}) -[:OWNS]->(t:OneToOneTarget {name: 't1'})");
+							tx.run("CREATE (s:OneToOneSource {name: 's2'})");
+							tx.commit();
+						}
+						return null;
+					}
+			);
+		}
+
+		private void assertOneToOneScenarioWithNulls(List<OneToOneSource.OneToOneSourceProjection> oneToOnes) {
+			assertThat(oneToOnes).hasSize(2);
+			assertThat(oneToOnes).extracting(OneToOneSource.OneToOneSourceProjection::getName).containsExactlyInAnyOrder("s1", "s2");
+			assertThat(oneToOnes).filteredOn(s -> s.getTarget() != null)
+					.extracting(s -> s.getTarget().getName()).containsExactly("t1");
+		}
+
+		@Test // GH-2305
+		void shouldFindOneToOneWithNullValues(@Autowired OneToOneRepository repository)  {
+			createOneToOneScenarioForNullValues();
+
+			List<OneToOneSource.OneToOneSourceProjection> oneToOnes = repository.findAllWithNullValues();
+			assertOneToOneScenarioWithNulls(oneToOnes);
+		}
 	}
 
 	@Nested
@@ -4118,6 +4145,9 @@ interface OneToOneRepository extends Neo4jRepository<OneToOneSource, String> {
 
 		@Query("MATCH (p1:#{#staticLabels})-[r:OWNS]-(p2) return *")
 		List<OneToOneSource> findAllWithCustomQueryReturnStar();
+
+		@Query("MATCH (p1:#{#staticLabels}) OPTIONAL MATCH (p1)-[r:OWNS]->(p2:OneToOneTarget) return p1, r, p2")
+		List<OneToOneSource.OneToOneSourceProjection> findAllWithNullValues();
 	}
 
 	interface RelationshipRepository extends Neo4jRepository<PersonWithRelationship, Long> {

src/test/java/org/springframework/data/neo4j/integration/shared/common/OneToOneSource.java

@@ -35,4 +35,13 @@ public class OneToOneSource {
 
 	@Relationship("OWNS")
 	private OneToOneTarget target;
+
+	/**
+	 * Simple DTO projection for OneToOneSource
+	 */
+	@Data
+	public static class OneToOneSourceProjection {
+		String name;
+		OneToOneTarget target;
+	}
 }