java - 将对象从对象 A 添加到对象 B 而不创建新对象？ hibernate

Question

假设我有两个对象，一个是用户对象，另一个是状态对象。状态对象基本上是美国的 50 个州，因此无需更改。然而，用户对象具有用户曾经所处的状态的集合。像这样:

@Entity
@Table(name="tbl_users")
class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name="id", unique=true, nullable = false)
    private int id;

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

    @OneToMany(targetEntity=State.class, orphanRemoval = false)
    @Column(name="states")
    private Collection<State> states;

    //getters and setters
}

States 实体如下所示:

@Entity
@Table(name="tbl_states")
class State {

   @Id
   @Column(name="id", unique=true, nullable=false)
   private int id;

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

   // getters and setters
}

添加用户的代码(使用 hibernate ):

public int addUser(User user) {
    em.persist(user);
    em.flush();
    return user.getId();
}

通过id获取状态的代码:

public State getStateById(int id) {
    return em.createQuery("SELECT s FROM State s WHERE s.id =:id, State.class)
            .setParameter("id", id)
            .getSingleResult();
}

但是当我尝试创建用户并选择多个状态时，我收到 PSQLException:

org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "uk_g6pr701i2pcq7400xrlb0hns"
2017-06-21T22:54:35.959991+00:00 app[web.1]:   Detail: Key (states_id)=(5) already exists.

我尝试查找 Cascade 方法，看看是否可以使用任何方法，但 Cascade.MERGE 和 Cascade.PERSIST 似乎做了同样的事情，其余的我认为我不需要(REMOVE、DETACH 等) 。我的问题是:如何向 User 对象添加状态而不出现该错误？

Answer 1

此代码有效:

class Example {

    @Test
    public void workingTest() {
        EntityManagerFactory emf = Persistence.createEntityManagerFactory("testPU");
        EntityManager em = emf.createEntityManager();

        // Creating three states
        State alabama = new State(state: 'Alabama');
        State louisiana = new State(state: 'Louisiana');
        State texas = new State(state: 'Texas');
        em.getTransaction().begin();
        em.persist(alabama);
        em.persist(louisiana);
        em.persist(texas);
        em.getTransaction().commit();

        List<State> states = em.createQuery('FROM State').getResultList();

        // Assert only three states on DB
        assert states.size() == 3;

        User userFromAlabama = new User();
        User userFromAlabamaAndTexas = new User();

        em.getTransaction().begin();
        State alabamaFromDB = em.find(State, alabama.getId());
        State texasFromDB = em.find(State, texas.getId());
        userFromAlabama.getStates().add(alabamaFromDB);
        userFromAlabamaAndTexas.getStates().add(alabamaFromDB);
        userFromAlabamaAndTexas.getStates().add(texasFromDB);

        em.persist(userFromAlabama);
        em.persist(userFromAlabamaAndTexas);
        em.getTransaction().commit();

        states = em.createQuery('FROM State').getResultList();

        // Assert only three states on DB again
        assert states.size() == 3;

        // Assert one user
        User userFromDB = em.find(User, userFromAlabama.getId());
        assert userFromDB.getStates().size() == 1;

        userFromDB = em.find(User, userFromAlabamaAndTexas.getId());
        assert userFromDB.getStates().size() == 2;
    }
}

@Entity
@Table(name="tbl_users")
class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id

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

    @ManyToMany
    private Collection<State> states = Lists.newArrayList()

    // Getters and setters
}

@Entity
@Table(name="tbl_states")
class State {

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

    @Column(name="state")
    private String state;
    // Getters and setters
}

您应该将映射更改为@ManyToMany!

数据库上必须有 3 个表，如下所示:TBL_USERS、TBL_STATES 和 TBL_USERS_TBL_STATES

TBL_USERS_TBL_STATES 表是 Hibernate 在使用 @ManyToMany 注释属性时使用的默认表名称。如果要更改 TBL_USERS_TBL_STATES 的表名，也可以使用 @JoinTable 注释。请参阅文档 here

使用此配置，您应该能够从数据库获取状态，将其添加到新用户，然后保留它。我做了一个单元测试，它有效!