Extract only certain fields using CrudRepository?

Question

I'm using CrudRepository to fetch Persons from database.

Is it possible to only fetch certain fields from the db, instead of the full entity, using a CrudRepository mapping method?

Example: I only want to extract all lastname column fields:

interface PersonRepository extends CrudRepository<Person, Long> {
    //of course this is invalid
    List<String> findAllLastname();
}

@Entity
public class Person {
    @Id
    private Long id;


    private String firstname, lastname;
}

findAllLastname() is of course not valid, but can I achieve this?

Answer 1

Giving a Query annotation with the specific SQL query should work

@Query("select p.lastName from Person  p")
List<Person> getAllLastName();

Answer 2

You can fetch specific field just doing this:

@Query("select p.lastName from Person p")
List<String> getAllLastName();

But sometimes this may not work (for example for enum type ). The best way here, IMO, is using projection as returned type, for example:

public interface PersonProjection {
    String getLastName();
}

@Query("select p.lastName as lastName from Person p")
List<PersonProjection> getAllPersonProjections();

In this case if you need to get, in the future, not only the lastName but also firstName then you simple update your PersonProjection and the query method:

public interface PersonProjection {
    String getFirstName();
    String getLastName();
}

@Query("select p.firstName as firstName, p.lastName as lastName from Person p")
List<PersonProjection> getAllPersonProjections();

Note that you should use aliases of the entity fields ( p.firstName as firstName ) that correspond to names of getters in the projection.