DATAGRAPH-1438 - Fix NPE when creating DTO based projections on empty result sets.

Author: michael-simons

src/main/java/org/springframework/data/neo4j/repository/query/DtoInstantiatingConverter.java

@@ -67,6 +67,10 @@ class DtoInstantiatingConverter implements Converter<EntityInstanceWithSource, O
 	@Override
 	public Object convert(EntityInstanceWithSource entityInstanceAndSource) {
 
+		if (entityInstanceAndSource == null) {
+			return null;
+		}
+
 		Object entityInstance = entityInstanceAndSource.getEntityInstance();
 		if (targetType.isInterface() || targetType.isInstance(entityInstance)) {
 			return entityInstance;

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

@@ -2838,7 +2838,7 @@ class Projection extends IntegrationTestBase {
 		void setupData(Transaction transaction) {
 			id1 = transaction.run(
 					"CREATE (n:PersonWithAllConstructor) "
-							+ "SET n.name = $name, n.sameValue = $sameValue, n.first_name = $firstName " + "RETURN id(n)",
+							+ "SET n.name = $name, n.sameValue = $sameValue, n.nullable = 'something', n.first_name = $firstName " + "RETURN id(n)",
 					Values.parameters("name", TEST_PERSON1_NAME, "sameValue", TEST_PERSON_SAMEVALUE, "firstName",
 							TEST_PERSON1_FIRST_NAME))
 					.next().get(0).asLong();
@@ -2867,6 +2867,21 @@ void mapsDtoProjectionWithDerivedFinderMethod(@Autowired PersonRepository reposi
 					.first().isEqualTo(TEST_PERSON1_FIRST_NAME);
 		}
 
+		@Test // DATAGRAPH-1438
+		void mapsOptionalDtoProjectionWithDerivedFinderMethod(@Autowired PersonRepository repository) {
+
+			assertThat(repository.findOneByFirstName(TEST_PERSON1_FIRST_NAME))
+					.map(DtoPersonProjection::getFirstName)
+					.hasValue(TEST_PERSON1_FIRST_NAME);
+			assertThat(repository.findOneByFirstName("foobar"))
+					.isEmpty();
+
+			assertThat(repository.findOneByNullable("something")).isNotNull()
+					.extracting(DtoPersonProjection::getFirstName)
+					.isEqualTo(TEST_PERSON1_FIRST_NAME);
+			assertThat(repository.findOneByNullable("foobar")).isNull();
+		}
+
 		@Test
 		void mapsInterfaceProjectionWithDerivedFinderMethodWithMultipleResults(@Autowired PersonRepository repository) {
 			assertThat(repository.findBySameValue(TEST_PERSON_SAMEVALUE)).hasSize(2);
@@ -2897,7 +2912,7 @@ void mapsInterfaceProjectionWithCustomQueryAndNodeReturnWithMultipleResults(
 		}
 
 		@Test
-		void mapsInterfaceProjectionWithCustomQueryAndNodeReturn2(@Autowired PersonRepository repository) {
+		void mapDtoProjectionWithCustomQueryAndNodeReturn(@Autowired PersonRepository repository) {
 
 			List<DtoPersonProjectionContainingAdditionalFields> projectedPeople = repository
 					.findAllDtoProjectionsWithAdditionalProperties(TEST_PERSON1_NAME);

src/test/java/org/springframework/data/neo4j/integration/imperative/repositories/PersonRepository.java

@@ -259,6 +259,10 @@ Optional<PersonWithAllConstructor> getOptionalPersonViaNamedQuery(@Param("part1"
 
 	List<DtoPersonProjection> findByFirstName(String firstName);
 
+	Optional<DtoPersonProjection> findOneByFirstName(String firstName);
+
+	DtoPersonProjection findOneByNullable(String nullable);
+
 	@Query(""
 			+ "MATCH (n:PersonWithAllConstructor) where n.name = $name "
 			+ "WITH n MATCH(m:PersonWithAllConstructor) WHERE id(n) <> id(m) "

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

@@ -19,6 +19,7 @@
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
 import static org.assertj.core.api.Assertions.tuple;
 
+import org.springframework.data.neo4j.integration.shared.common.DtoPersonProjection;
 import org.springframework.data.neo4j.integration.shared.common.SimplePerson;
 import org.springframework.data.neo4j.integration.shared.common.ThingWithFixedGeneratedId;
 import reactor.core.publisher.Flux;
@@ -2144,6 +2145,16 @@ void mapsInterfaceProjectionWithCustomQueryAndNodeReturnWithMultipleResults(
 			StepVerifier.create(repository.loadAllProjectionsWithNodeReturn()).expectNextCount(2).verifyComplete();
 		}
 
+		@Test // DATAGRAPH-1438
+		void mapsOptionalDtoProjectionWithDerivedFinderMethod(@Autowired ReactivePersonRepository repository) {
+
+			StepVerifier.create(repository.findOneByFirstName(TEST_PERSON1_FIRST_NAME).map(DtoPersonProjection::getFirstName))
+					.expectNext(TEST_PERSON1_FIRST_NAME)
+					.verifyComplete();
+
+			StepVerifier.create(repository.findOneByFirstName("foobar").map(DtoPersonProjection::getFirstName))
+					.verifyComplete();
+		}
 	}
 
 	@Nested

src/test/java/org/springframework/data/neo4j/integration/reactive/repositories/ReactivePersonRepository.java

@@ -80,4 +80,6 @@ public interface ReactivePersonRepository extends ReactiveNeo4jRepository<Person
 
 	@Query("MATCH (n:PersonWithAllConstructor) return n")
 	Flux<PersonProjection> loadAllProjectionsWithNodeReturn();
+
+	Mono<DtoPersonProjection> findOneByFirstName(String firstName);
 }