Select one column using Spring Data JPA

Question

Does anyone have any idea how to get a single column using Spring Data JPA? I created a repository like below in my Spring Boot project, but always get the {"cause":null,"message":"PersistentEntity must not be null!"} error when accessing the Restful URL.

@RepositoryRestResource(collectionResourceRel = "users", path = "users")
public interface UsersRepository extends CrudRepository<Users, Integer> {

    @Query("SELECT u.userName  FROM Users u")
    public List<String> getUserName();
}

Then if I access the Restful URL like ../users/search/getUserName, I get the error: {"cause":null,"message":"PersistentEntity must not be null!"}

Answer 1

Create a Projection interface

public interface UserNameOnly {
    String getUserName();
}

Then in your repository interface return that type instead of the user type

public interface UserRepository<User> extends JpaRepository<User,String> {
    List<UsernameOnly> findNamesByUserNameNotNull();
}

The get method in the projection interface must match a get method of the defined type on the JPA repository, in this case User. The "findBySomePropertyOnTheObjectThatIsNotNull" allows you to get a List of the entities (as opposed to an Iterable) based on some criteria, which for a findAll can simply be if the unique identifier (or any other NonNull field) is not null.