Hibernate：使用两个表进行多对一映射

Question

I have two tables Users(id,name,email,role_id) and role(id,type) I am using @ManyToOne (cascade=CascadeType.ALL) role in user bean Because of it a new role is getting inserted every time. But I want user to use existing entry of role table. Please help

Answer 1

You need to find the existing Role object first and then set it into User object.

In other words, the Role object needs to be already created and managed before you save User.

Answer 2

I suppose

@Entity
public class Users{
  @Id
  private int id;
  private String email;
  @ManyToOne(cascade=CascadeType.ALL)
  private Role role_id;
}

@Entity
public class Role{
    @Id
    private int id;
    private String type;
 } 

Using JPA

Create roles first

Role role1 = new Role(1,"Role1");
Role role2 = new Role(2,"Role2");

Then add the role to the user

User user1 = new User(1,"User1","mail",role1);

EntityManager em = ....
em.persist(user1);

As we have cascade option role1 and user1 will be saved in the database, dont forget to use a transaction to do this operation

If you want to use a pre-existing Role consider retrieve it first from database

Role existingRole = em.find(Role.class, 1);
User user1 = new User(1,"User1","mail",existingRole );

EntityManager em = ....
em.persist(user1);

Answer 3

Keep a separate @Column for role_id in Users apart from the one annotated with @ManyToOne then set the role_id to Users and persist Users. You dont need a cascade so remove it to have casacade type as none rather than ALL. In the @ManyToOne you should give insertable=false & updatable=false to avoid repeated column issue.

See this example

Answer 4

考虑到用户和角色中的id列都是主键，您需要的是当持久新用户首先通过数据库中的类型检索所需角色，然后在用户实体中设置该角色并保留该用户时。