JPA: Join Fetch results to NULL on empty many side

问题

I have a one to many relationship between User and GameMap. One user can have many maps.

User class:

// LAZY LOADED
@OneToMany(cascade = CascadeType.ALL, mappedBy = "creater")
private final List<GameMap> maps = new ArrayList<>();

However, sometimes I need to eager load the maps. To avoid the LazyInitializationException after closing Session, I have two variants of retrieving Users.

User Repository:

public interface UserRepository extends JpaRepository<User, Long> {

    Optional<User> findById( Long id );

    @Query("SELECT u FROM User u JOIN FETCH u.maps WHERE u.id = (:id)")
    public User findByIdEagerFetch( @Param("id") Long id );
}

Problem:
However, the JPQL JOIN FETCH variant to eager load in one go the user and his maps returns a NULL user if there are no maps for this user in the table.

Question:
How can I rewrite the JPQL statement in order to retrieve the user and optionally(!) all his maps but if there are no maps, than thats okay, but dont return a NULL user.

回答1:

A FETCH JOIN actually will resolve to an inner join in SQL. This means that any records/entities in the User table which have no maps will be removed from the result set. You need the LEFT keyword on your FETCH JOIN to get all the results, even those without a map.

@Query("SELECT u FROM User u LEFT JOIN FETCH u.maps WHERE u.id = (:id)")
public User findByIdEagerFetch( @Param("id") Long id );

来源：https://stackoverflow.com/questions/43879474/jpa-join-fetch-results-to-null-on-empty-many-side