java - 如何对可以为空的列使用 hibernate 延迟加载

Question

这是我的实体:

@Entity
@Table(name = "users")
public class User {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;

@Column(name = "name")
private String name;

@Column(name = "surname")
private String surname;


@ManyToOne(fetch = FetchType.LAZY, cascade=CascadeType.MERGE)
@JoinColumn(name = "id_city")
private City city;
//...

}

在我的存储库中我有:

public interface UserRepository extends JpaRepository<User, Long>{

@Query("SELECT u FROM User u JOIN FETCH u.city")
public List<User> findAllUserForApi();

}

如果表中有任何城市，findAllUserForApi() 会显示有关用户的完整信息:

[{"id":1,"name":"John","surname":"Pillman","city":{"id":1,"name":"New York"}]

如果没有城市，我想至少得到 [{"id":1,"name":"John","surname":"Pillman","city":null]但是我什么都没有:[]

请帮帮我。

Answer 1

鉴于您已经在使用自定义查询，最简单的解决方案是LEFT JOIN FETCH:
@Query("SELECT u FROM User u LEFT JOIN FETCH u.city")
这样无论有没有城市，所有用户都会被加载；对于那些拥有城市的人，它将通过 user.getCity() 获得。