Apply DTO projection through JDBC's Query by Example.

Spring Data JDBC doesn't allow projections through JdbcAggregateOperations yet and so we need to apply DTO conversion.

Closes #2098

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/support/FetchableFluentQueryByExample.java

@@ -32,6 +32,7 @@ import org.springframework.data.domain.ScrollPosition;
 import org.springframework.data.domain.Sort;
 import org.springframework.data.domain.Window;
 import org.springframework.data.jdbc.core.JdbcAggregateOperations;
+import org.springframework.data.projection.ProjectionFactory;
 import org.springframework.data.relational.core.query.Query;
 import org.springframework.data.relational.repository.query.RelationalExampleMapper;
 import org.springframework.util.Assert;
@@ -47,19 +48,23 @@ class FetchableFluentQueryByExample<S, R> extends FluentQuerySupport<S, R> {
 
 	private final RelationalExampleMapper exampleMapper;
 	private final JdbcAggregateOperations entityOperations;
+	private final ProjectionFactory projectionFactory;
 
 	FetchableFluentQueryByExample(Example<S> example, Class<R> resultType, RelationalExampleMapper exampleMapper,
-			JdbcAggregateOperations entityOperations) {
-		this(example, Sort.unsorted(), 0, resultType, Collections.emptyList(), exampleMapper, entityOperations);
+			JdbcAggregateOperations entityOperations, ProjectionFactory projectionFactory) {
+		this(example, Sort.unsorted(), 0, resultType, Collections.emptyList(), exampleMapper, entityOperations,
+				projectionFactory);
 	}
 
 	FetchableFluentQueryByExample(Example<S> example, Sort sort, int limit, Class<R> resultType,
-			List<String> fieldsToInclude, RelationalExampleMapper exampleMapper, JdbcAggregateOperations entityOperations) {
+			List<String> fieldsToInclude, RelationalExampleMapper exampleMapper, JdbcAggregateOperations entityOperations,
+			ProjectionFactory projectionFactory) {
 
-		super(example, sort, limit, resultType, fieldsToInclude);
+		super(example, sort, limit, resultType, fieldsToInclude, projectionFactory, entityOperations.getConverter());
 
 		this.exampleMapper = exampleMapper;
 		this.entityOperations = entityOperations;
+		this.projectionFactory = projectionFactory;
 	}
 
 	@Override
@@ -167,6 +172,6 @@ class FetchableFluentQueryByExample<S, R> extends FluentQuerySupport<S, R> {
 			List<String> fieldsToInclude) {
 
 		return new FetchableFluentQueryByExample<>(example, sort, limit, resultType, fieldsToInclude, this.exampleMapper,
-				this.entityOperations);
+				this.entityOperations, this.projectionFactory);
 	}
 }

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/support/FluentQuerySupport.java

@@ -21,9 +21,12 @@ import java.util.List;
 import java.util.function.Function;
 
 import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.data.convert.DtoInstantiatingConverter;
 import org.springframework.data.domain.Example;
 import org.springframework.data.domain.Sort;
-import org.springframework.data.projection.SpelAwareProxyProjectionFactory;
+import org.springframework.data.projection.EntityProjection;
+import org.springframework.data.projection.ProjectionFactory;
+import org.springframework.data.relational.core.conversion.RelationalConverter;
 import org.springframework.data.repository.query.FluentQuery;
 import org.springframework.util.Assert;
 
@@ -41,16 +44,19 @@ abstract class FluentQuerySupport<S, R> implements FluentQuery.FetchableFluentQu
 	private final int limit;
 	private final Class<R> resultType;
 	private final List<String> fieldsToInclude;
+	private final ProjectionFactory projectionFactory;
+	private final RelationalConverter converter;
 
-	private final SpelAwareProxyProjectionFactory projectionFactory = new SpelAwareProxyProjectionFactory();
-
-	FluentQuerySupport(Example<S> example, Sort sort, int limit, Class<R> resultType, List<String> fieldsToInclude) {
+	FluentQuerySupport(Example<S> example, Sort sort, int limit, Class<R> resultType, List<String> fieldsToInclude,
+			ProjectionFactory projectionFactory, RelationalConverter converter) {
 
 		this.example = example;
 		this.sort = sort;
 		this.limit = limit;
 		this.resultType = resultType;
 		this.fieldsToInclude = fieldsToInclude;
+		this.projectionFactory = projectionFactory;
+		this.converter = converter;
 	}
 
 	@Override
@@ -118,8 +124,18 @@ abstract class FluentQuerySupport<S, R> implements FluentQuery.FetchableFluentQu
 			return (Function<Object, R>) Function.identity();
 		}
 
-		if (targetType.isInterface()) {
-			return o -> projectionFactory.createProjection(targetType, o);
+		EntityProjection<?, ?> entityProjection = converter.introspectProjection(targetType, inputType);
+
+		if (entityProjection.isProjection()) {
+
+			if (targetType.isInterface()) {
+				return o -> projectionFactory.createProjection(targetType, o);
+			}
+
+			DtoInstantiatingConverter dtoConverter = new DtoInstantiatingConverter(targetType, converter.getMappingContext(),
+					converter.getEntityInstantiators());
+
+			return o -> (R) dtoConverter.convert(o);
 		}
 
 		return o -> DefaultConversionService.getSharedInstance().convert(o, targetType);

spring-data-jdbc/src/main/java/org/springframework/data/jdbc/repository/support/SimpleJdbcRepository.java

@@ -26,6 +26,7 @@ import org.springframework.data.domain.Sort;
 import org.springframework.data.jdbc.core.JdbcAggregateOperations;
 import org.springframework.data.jdbc.core.convert.JdbcConverter;
 import org.springframework.data.mapping.PersistentEntity;
+import org.springframework.data.projection.SpelAwareProxyProjectionFactory;
 import org.springframework.data.relational.repository.query.RelationalExampleMapper;
 import org.springframework.data.repository.CrudRepository;
 import org.springframework.data.repository.PagingAndSortingRepository;
@@ -48,6 +49,7 @@ import org.springframework.util.Assert;
 public class SimpleJdbcRepository<T, ID>
 		implements CrudRepository<T, ID>, PagingAndSortingRepository<T, ID>, QueryByExampleExecutor<T> {
 
+	private final SpelAwareProxyProjectionFactory projectionFactory = new SpelAwareProxyProjectionFactory();
 	private final JdbcAggregateOperations entityOperations;
 	private final PersistentEntity<T, ?> entity;
 	private final RelationalExampleMapper exampleMapper;
@@ -197,7 +199,7 @@ public class SimpleJdbcRepository<T, ID>
 		Assert.notNull(queryFunction, "Query function must not be null");
 
 		FluentQuery.FetchableFluentQuery<S> fluentQuery = new FetchableFluentQueryByExample<>(example,
-				example.getProbeType(), this.exampleMapper, this.entityOperations);
+				example.getProbeType(), this.exampleMapper, this.entityOperations, this.projectionFactory);
 
 		return queryFunction.apply(fluentQuery);
 	}

spring-data-jdbc/src/test/java/org/springframework/data/jdbc/repository/JdbcRepositoryIntegrationTests.java

@@ -1236,6 +1236,32 @@ public class JdbcRepositoryIntegrationTests {
 		assertThat(matches).isEqualTo(2);
 	}
 
+	@Test // GH-2098
+	void projectByExample() {
+
+		String searchName = "Diego";
+		Instant now = Instant.now().truncatedTo(ChronoUnit.MILLIS);
+
+		DummyEntity entity = createEntity();
+
+		entity.setName(searchName);
+		entity.setPointInTime(now.minusSeconds(10000));
+		entity = repository.save(entity);
+
+		record DummyProjection(String name) {
+
+		}
+
+		Example<DummyEntity> example = Example.of(createEntity(searchName, it -> it.setBytes(null)));
+
+		DummyProjection projection = repository.findBy(example,
+				p -> p.project("name").as(DummyProjection.class).firstValue());
+		assertThat(projection.name()).isEqualTo(entity.name);
+
+		projection = repository.findBy(example, p -> p.project("flag").as(DummyProjection.class).firstValue());
+		assertThat(projection.name()).isNull();
+	}
+
 	@Test // GH-1192
 	void fetchByExampleFluentOnlyInstantFirstSimple() {
 
@@ -2005,6 +2031,7 @@ public class JdbcRepositoryIntegrationTests {
 
 	static class DummyEntity {
 
+		@Id Long idProp;
 		String name;
 		Instant pointInTime;
 		OffsetDateTime offsetDateTime;
@@ -2012,7 +2039,6 @@ public class JdbcRepositoryIntegrationTests {
 		AggregateReference<DummyEntity, Long> ref;
 		Direction direction;
 		byte[] bytes = new byte[] { 0, 0, 0, 0, 0, 0, 0, 0 };
-		@Id private Long idProp;
 
 		public DummyEntity(String name) {
 			this.name = name;