java - JPA更新多对多删除记录

Question

我在两个实体之间有一个@ManyToMany 关系。当我在拥有方执行更新时，JPA 似乎从我的数据库中删除了所有链接记录并重新插入它们。对我来说这是个问题，因为我有一个 MySQL 触发器在删除记录之前触发。关于如何解决这个问题的任何想法？

@Entity
public class User {

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

    ...

    @ManyToMany
    @JoinTable(name="groups", joinColumns=
        @JoinColumn(name="username", referencedColumnName="username"),
            inverseJoinColumns=@JoinColumn(name="groupname",
                    referencedColumnName="type_id"))
    private List<UserType> types;

    ...

}

@Entity
public class UserType {

    @Id
    @Column(name="type_id")
    private String id;

    @ManyToMany(mappedBy="types")
    private List<User> users;

    ...
} 

Answer 1

使用 Set 而不是 List 解决了这个问题。但我不知道它为什么有效。

Hibernate 提供的另一种解决方案是将@ManyToMany 关联拆分为两个双向@OneTo@Many 关系。参见 Hibernate 5.2 documentation例如。

If a bidirectional @OneToMany association performs better when removing or changing the order of child elements, the @ManyToMany relationship cannot benefit from such an optimization because the foreign key side is not in control. To overcome this limitation, the link table must be directly exposed and the @ManyToMany association split into two bidirectional @OneToMany relationships.