Spring + JPA有很多很多关系

Question

我是Spring的新手,我正在尝试按照我的预期建立多对多的关系.关系工作正常,创建表并正确插入数据.我期望的是,当我清空List(即我从"Group"类型的对象中清空ArrayList"users")时,我希望系统从数据库中删除关系 - 但事实并非如此.

我有以下课程:

@Entity
@Table(name = "`group`")
public class Group
{
    @Id
    @GeneratedValue
    @Column(name = "id")
    private int id;

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

    @ManyToMany(cascade = {CascadeType.ALL})
    @JoinTable(
            name = "`user_has_group`",
            joinColumns = @JoinColumn(name = "group_id", referencedColumnName = "id"),
            inverseJoinColumns = @JoinColumn(name = "user_id", referencedColumnName = "id")
    )
    private List<User> users = new ArrayList<User>();

    ...
}

@Entity
@Table(name = "`user`")
public class User
{
    @Id
    @GeneratedValue
    @Column(name = "id")
    private int id;

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

    @ManyToMany(mappedBy = "users")
    private List<Group> groups = new ArrayList<Group>();

    ...
}

以下是DAO:

@Repository
public class GroupJpaDao implements GroupDao
{
    private EntityManager em;

    @Transactional
    public void save(Group group)
    {
        this.em.merge(group);
    }

    ...

    @PersistenceContext
    void setEntityManager(EntityManager entityManager)
    {
        this.em = entityManager;
    }
}

@Repository
public class UserJpaDao implements UserDao
{
    private EntityManager em;

    @Transactional
    public void save(User user)
    {
        this.em.merge(user);
    }

    ...

    @PersistenceContext
    void setEntityManager(EntityManager entityManager)
    {
        this.em = entityManager;
    }
}

这是测试方法:

@Test
public void test()
{
    Group g = new Group();
    g.setName("Test group");
    groupDao.save(g); // Works fine - inserts the group into the database

    User u = new User();
    u.setName("Test user");
    userDao.save(u); // Works fine - inserts the user into the database

    g.addUser(u);
    groupDao.save(g); // Works fine - adds the many-to-many relationship into the database

    g.removeAllUsers();
    groupDao.save(g); // Doesn't work - I'm expecting it to remove all the related relationships from the database but it doesn't!
}

我做错了什么或是不可能的事情？

任何暗示都非常感激.

谢谢!

Answer 1

我重读了你的问题，现在答案很明确了。您创建一个 Groupg并保存它。但由于您的 save 方法使用merge，并且您没有考虑将merge其分配给 的返回值g，因此您不断合并瞬态组 g，该组从未分配任何 ID。因此，每次调用 merge 时，您实际上都会创建一个新组，而不是修改之前创建的组。

将保存方法更改为

public Group save(Group group)
{
    return this.em.merge(group);
}

并始终将结果重新分配给g：

g = groupDao.save(g);

当然，用户也必须这样做。