Document that simple base-repository method overrides cannot return projections

[mereshow]

Even if an interface projection (DTO) is used as the return type of the findAll() method of a repository, the method returns the entity associated with the repository, instead of the DTO.

I create an entity (reduced for compactness):

@Entity
@Table(name = "arpsi")
public class Arpsi {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String nombre;
    private String codigo;
    @JsonRawValue
    private String dph;
    // getters and setters not shown
}

I create a DTO with fewer fields:

public interface ArpsiDTO {
    public Long getId();
    public String getNombre();
}

I create a repository:

public interface ArpsiRepository extends Repository<Arpsi, Long> {
    Iterable<ArpsiDTO> findById();
    Iterable<ArpsiDTO> findAll();
    ...
}

I use a service that uses the repository to retrieve the data. I use ArpsiDTO as the return type in every step, but the service gets the Entity instead of the DTO (with errors due to lazy initialization of a collection).

I tested it with several repositories, and only methods that include any condition (findByX or similar) return the DTO.
Converting the ArpsiDTO to a class, and using a constructor based projection with a custom query for the findAll() method works as expected (returns the DTO):

@Query("SELECT new com.equixlab.api.model.augas.arpsi.ArpsiDTO(a.id, a.nombre, a.codigo) FROM Arpsi a")
	Iterable<ArpsiDTO> findAll();

It also works if we create a method with an always true condition, such as:

Iterable<ArpsiDTO> findAllByIdNotNull();

I suppose this happens with all the methods that exist in the CrudRepository interface and expose in a Repository interface (inspecting the CrudRepository interface, the methods use <T> or <S extends T>, so a DTO cannot be used/returned).

Is this by design? I could not find anything in the docs.

[mp911de]

findAll is a method that is defined in CrudRepository and implemented in SimpleJpaRepository. Without @Query, findAll maps to SimpleJpaRepository.findAll() which is a regular Java method that isn't aware of the repository definition (and it can't ever be).

Therefore projections do not work with implementation methods.

Adding @Query turns the method into a query method using query derivation and because the query mechanism is different, the method gets enabled for projections. We should mention this in our documentation.

[mereshow]

findAll is a method that is defined in CrudRepository and implemented in SimpleJpaRepository. Without @Query, findAll maps to SimpleJpaRepository.findAll() which is a regular Java method that isn't aware of the repository definition (and it can't ever be).

Therefore projections do not work with implementation methods.

Adding @Query turns the method into a query method using query derivation and because the query mechanism is different, the method gets enabled for projections. We should mention this in our documentation.

This is what I deducted after reviewing the CrudRepository interface. Adding this explanation to the documentation would help newcomers like me.
Thank you!