Defer PersistentEntity lookup until actual DTO conversion.

We now lazily instantiate DtoInstantiatingConverter to defer the entity lookup if needed.

Closes #612

Author: mp911de

src/main/java/org/springframework/data/r2dbc/repository/query/R2dbcQueryExecution.java

@@ -29,6 +29,7 @@
 import org.springframework.data.relational.repository.query.DtoInstantiatingConverter;
 import org.springframework.data.repository.query.ResultProcessor;
 import org.springframework.data.repository.query.ReturnedType;
+import org.springframework.data.util.Lazy;
 import org.springframework.data.util.ReflectionUtils;
 import org.springframework.r2dbc.core.FetchSpec;
 import org.springframework.util.ClassUtils;
@@ -73,13 +74,16 @@ final class ResultProcessingConverter implements Converter<Object, Object> {
 		private final ResultProcessor processor;
 		private final MappingContext<? extends RelationalPersistentEntity<?>, ? extends RelationalPersistentProperty> mappingContext;
 		private final EntityInstantiators instantiators;
+		private final Lazy<DtoInstantiatingConverter> converter;
 
 		ResultProcessingConverter(ResultProcessor processor,
 				MappingContext<? extends RelationalPersistentEntity<?>, ? extends RelationalPersistentProperty> mappingContext,
 				EntityInstantiators instantiators) {
 			this.processor = processor;
 			this.mappingContext = mappingContext;
 			this.instantiators = instantiators;
+			this.converter = Lazy.of(() -> new DtoInstantiatingConverter(processor.getReturnedType().getReturnedType(),
+					this.mappingContext, this.instantiators));
 		}
 
 		/* (non-Javadoc)
@@ -109,10 +113,7 @@ public Object convert(Object source) {
 				}
 			}
 
-			Converter<Object, Object> converter = new DtoInstantiatingConverter(returnedType.getReturnedType(),
-					this.mappingContext, this.instantiators);
-
-			return this.processor.processResult(source, converter);
+			return this.processor.processResult(source, it -> this.converter.get().convert(it));
 		}
 	}
 }

src/test/java/org/springframework/data/r2dbc/repository/query/StringBasedR2dbcQueryUnitTests.java

@@ -19,7 +19,15 @@
 import static org.mockito.ArgumentMatchers.*;
 import static org.mockito.Mockito.*;
 
+import io.r2dbc.spi.test.MockColumnMetadata;
+import io.r2dbc.spi.test.MockResult;
+import io.r2dbc.spi.test.MockRow;
+import io.r2dbc.spi.test.MockRowMetadata;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
+
 import java.lang.reflect.Method;
+import java.time.LocalDate;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
@@ -38,6 +46,7 @@
 import org.springframework.data.r2dbc.dialect.PostgresDialect;
 import org.springframework.data.r2dbc.mapping.R2dbcMappingContext;
 import org.springframework.data.r2dbc.repository.Query;
+import org.springframework.data.r2dbc.testing.StatementRecorder;
 import org.springframework.data.relational.core.mapping.RelationalMappingContext;
 import org.springframework.data.repository.Repository;
 import org.springframework.data.repository.core.RepositoryMetadata;
@@ -274,6 +283,29 @@ void usesDtoTypeForDtoResultMapping() {
 		assertThat(query.resolveResultType(query.getQueryMethod().getResultProcessor())).isEqualTo(PersonDto.class);
 	}
 
+	@Test // gh-612
+	void selectsSimpleType() {
+
+		MockRowMetadata metadata = MockRowMetadata.builder()
+				.columnMetadata(MockColumnMetadata.builder().name("date").build()).build();
+		LocalDate value = LocalDate.now();
+		MockResult result = MockResult.builder().rowMetadata(metadata)
+				.row(MockRow.builder().identified(0, LocalDate.class, value).build()).build();
+
+		StatementRecorder recorder = StatementRecorder.newInstance();
+		recorder.addStubbing(s -> s.equals("SELECT MAX(DATE)"), result);
+
+		databaseClient = DatabaseClient.builder() //
+				.connectionFactory(recorder) //
+				.bindMarkers(PostgresDialect.INSTANCE.getBindMarkersFactory()).build();
+
+		StringBasedR2dbcQuery query = getQueryMethod("findAllLocalDates");
+
+		Flux<Object> flux = (Flux) query.execute(new Object[0]);
+
+		flux.as(StepVerifier::create).expectNext(value).verifyComplete();
+	}
+
 	private StringBasedR2dbcQuery getQueryMethod(String name, Class<?>... args) {
 
 		Method method = ReflectionUtils.findMethod(SampleRepository.class, name, args);
@@ -328,6 +360,9 @@ private interface SampleRepository extends Repository<Person, String> {
 
 		@Query("SELECT * FROM person")
 		PersonProjection findAsInterfaceProjection();
+
+		@Query("SELECT MAX(DATE)")
+		Flux<LocalDate> findAllLocalDates();
 	}
 
 	static class PersonDto {