mysql - 当我更新多对多关系中的实体时，如何阻止 Hibernate/JPA 删除连接表记录？

Question

我想更新实体而不更新/编辑子实体。这是有问题的实体:

@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private int id;
    
    //other fields omitted for brevity
    
    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "project_user",
    joinColumns = @JoinColumn(name="user_id", updatable = false),
    inverseJoinColumns = @JoinColumn(name="project_id",updatable = false))
    @JsonIgnore
    private Set<Project> projects;
    
    //getters and setters omitted for brevity

    @Override
    public int hashCode() {
        return id;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        User other = (User) obj;
        if (id != other.id)
            return false;
        return true;
    }
    
}

这是项目实体:

@Entity
@Table(name = "project")
public class Project {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private int id;
    
    //other fields omitted for brevity
    
    @ManyToMany
    @JoinTable(name = "project_user",
    joinColumns = @JoinColumn(name="project_id"),
    inverseJoinColumns = @JoinColumn(name="user_id"))
    @JsonIgnoreProperties(value = {"projects", "connected", "notifications"})
    private Set<User> members;

    @Override
    public int hashCode() {
        return id;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Project other = (Project) obj;
        if (id != other.getId())
            return false;
        return true;
    }
    
}

这是合并过程:

@Override
    public void update(User user) {
        //user is created from a dto as a new object (that has its id set manually)
        entityManager.merge(user);
    }

问题是 hibernate 日志从连接表中删除。我怎样才能阻止它？

hibernate 日志

Hibernate: //login query
    select
        distinct authuser0_.id as id1_7_,
        authuser0_.enabled as enabled8_7_,
        authuser0_.password as password5_7_,
        authuser0_.username as username6_7_,
        roles1_.username as username1_0_0__,
        roles1_.authority as authorit2_0_0__ 
    from
        users authuser0_ 
    inner join
        authorities roles1_ 
            on authuser0_.username=roles1_.username 
    where
        authuser0_.username=?
Hibernate: //entityManager.merge() calls this to merge it
    select 
        user0_.id as id1_7_0_,
        user0_.email as email2_7_0_,
        user0_.first_name as first_na3_7_0_,
        user0_.image as image7_7_0_,
        user0_.last_name as last_nam4_7_0_,
        user0_.password as password5_7_0_,
        user0_.username as username6_7_0_ 
    from
        users user0_ 
    where
        user0_.id=?
Hibernate: //the actual update statement
    update 
        users 
    set
        email=?,
        first_name=?,
        image=?,
        last_name=?,
        password=?,
        username=? 
    where
        id=?
Hibernate: // not wanted, need to stop this query
    delete 
    from
        project_user 
    where
        user_id=?

我在网上读到它与 equals 和 hashcode 有关。但是，我似乎无法让它发挥作用。

我知道两个选项都不是最佳的:

1. 编写 native 查询而不是合并方法。问题:不可扩展，每次更改都需要手动编辑
2. 为表创建一个新实体(因此该表将有 2 个映射到它的实体)，而不附加到其他表。问题:重复的代码和实体

提前致谢

Answer 1

嗯，我知道您已经接受了答案，但是......

那里发生的事情比你想象的要多。

出现 Hibernate: delete from project_user where user_id=? 的原因是之后有插入。你没看到他们吗？如果该用户的关系表被清空但后来没有插入，那么关系可能没有更新？也只能重现删除操作，但您应该测试您的代码并验证该用户没有项目。这可能意味着您没有更新合并用户中的项目字段。

因此，运行一个基本的 JPA 示例，让我们从创建一个起点开始解决您的问题:

    em.getTransaction().begin();
    Project project = new Project();
    em.persist(project);
    User user = new User();
    Set<Project> projects = new HashSet<>();
    projects.add(project);
    user.setProjects(projects);
    em.persist(user);
    em.getTransaction().commit();

然后，清除entityManager上下文:

    em.clear();

然后重现您的问题:

    em.getTransaction().begin();
    Project p2 = new Project();
    em.persist(p2);
    projects.add(p2);
    User un = em.find(User.class, 1);
    un.setProjects(projects);
    em.merge(un);
    em.getTransaction().commit();

第二部分给我日志:

Hibernate: insert into project (id) values (default)
Hibernate: select user0_.id as id1_2_0_ from users user0_ where user0_.id=?
Hibernate: select project0_.id as id1_0_0_ from project project0_ where project0_.id=?
Hibernate: delete from project_user where user_id=?
Hibernate: insert into project_user (user_id, project_id) values (?, ?)
Hibernate: insert into project_user (user_id, project_id) values (?, ?)

现在让我们通过更改代码并再次运行来删除有问题的 delete 语句:

    Project p2 = new Project();
    em.persist(p2);
    User un = em.find(User.class, 1);
    projects = un.getProjects();
    projects.add(p2);
    em.merge(un);
    em.getTransaction().commit();

这给了我日志:

Hibernate: insert into project (id) values (default)
Hibernate: select user0_.id as id1_2_0_ from users user0_ where user0_.id=?
Hibernate: select projects0_.user_id as user_id1_1_0_, projects0_.project_id as project_2_1_0_, project1_.id as id1_0_1_ from project_user projects0_ inner join project project1_ on projects0_.project_id=project1_.id where projects0_.user_id=?
Hibernate: insert into project_user (user_id, project_id) values (?, ?)

没有删除语句。仔细查看第一个“第二部分”，请注意我将新项目添加到了原始项目集中，该项目不再是 JPA 持久性上下文的一部分。在第二个示例中，我专门从属于 JPA 持久性上下文的 user 对象中检索了一组项目。现在，当我更新项目集并合并用户 JPA 时，不必重新开始处理用户/项目关系。

顺便说一句:就 mappedBy 逻辑而言，接受的答案是准确的，但此代码是根据您的原始示例完成的。只要 JPA 知道您在做什么，它就会正确处理它。当您在 JPA 之外执行操作时，JPA 将“下注”并重新创建。