JPA：在很多方面将结果连接到NULL结果 [英] JPA: Join Fetch results to NULL on empty many side

问题描述

我在User和GameMap之间有一对多的关系。一位用户可以有多个地图。

用户类别：

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

但是，有时我需要加载地图。为了避免关闭Session之后的 LazyInitializationException ，我有两种检索用户的变体。

p>

  public interface UserRepository扩展JpaRepository< User，Long> {
 
可选< 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 ）; 
} 
  

问题：

但是，JPQL JOIN FETCH变体预先加载到用户，并且如果表中没有此用户的地图，则其地图将返回NULL用户。

问题：

如何重写JPQL语句以检索用户和（可选）（！）所有地图，但如果没有地图，

解决方案

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

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.

解决方案

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

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