Filtering with Spring Data Projection - spring

Question

I've created a class based Projection for my Spring Data Repository. That works great. Then I tried to annotate the constructor with #QueryProjection from QueryDSL, hoping to get a REST Endpoint with paging, sorting and filtering.
The code looks like this, but there are way more fields and details omitted for brevity:
Entity:
#Data
#Entity
public class Entity extends BaseEntity {
private String fieldA, fieldB;
private AnotherEntity ae;
}
DTO:
#Getter
#FieldDefaults(makeFinal = true, level = AccessLevel.PRIVATE)
public class EntityDto {
private final String fieldA;
private final String anotherEntityFieldC;
#QueryProjection
public EntityDto(final String fieldA, final String anotherEntityFieldC) {
this.fieldA = fieldA;
this.anotherEntityFieldC = anotherEntityFieldC;
}
}
Repository:
public EntityRepo extends JpaRepository<Entity, Long>, QuerydslBinderCustomizer<EntityPath<Entity>>, QuerydslPredicateExecutor<Entity> {
Page<EntityDto> findPageProjectedBy(Predicate predicate, Pageable pageable);
}
Endpoint:
#RestController
#RequestMapping(EntityEndpoint.ROOT)
#RequiredArgsConstructor(onConstructor = #__({#Autowired}))
public EntityEndpoint {
private final EntityRepo er;
#GetMapping
public ResponseEntity<PagedResources<Resource<EntityDto>>> getAllEntities(
Pageable pageable,
#QuerydslPredicate(root = EntityDto#.class) Predicate predicate,
PagedResourcesAssembler<EntityDto> assembler) {
Page<EntityDto> page = er.findPageProjectedBy(predicate, pageable);
return new ResponseEntity<>(assembler.toResource(page), HttpStatus.OK);
}
}
I get the exception:
java.lang.IllegalStateException: Did not find a static field of the same type in class at.dataphone.logis4.model.dto.QBuchungDto!
Stacktrace as gist
And that's the URL:
curl -X GET --header 'Accept: application/json' 'http://localhost:8080/Entity'

Answer 1

Well, it seems like you can only query with the actual Entity.
I think of it as the part in the 'WHERE'-clause, which can only have columns actually in the selected Entities, while the DTO projection happens in the 'SELECT'-clause.
The #QueryProjection-Annotation seems to serve only for the QueryDsl code generator to mark class which then can be used to create projections in a select-clause