使用CrudRepository仅提取某些字段？

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.