如何使用Hibernate管理两个表中的多对多关系

Question

I am using hibernate-jpa-2.0-api 1.0.0.Final and hibernate 3.5.5-Final .

I have User and Role entities each having many-to-many relationships to each other (bidirectional).

The relationships create total four table I have reduce to three tables.

But still my requirement not fulfilled, I just want to manage the data in two tables.

Answer 1

You can manage it like this:

@ManyToMany(cascade = {
        CascadeType.ALL
    })
    @JoinTable(name = "NAME OF YOUR TABLE WHERE RELATIONS ARE STORED",
        joinColumns = @JoinColumn(name = "ID_FIRST_TABLE"),
        inverseJoinColumns = @JoinColumn(name = "ID_SECOND_TABLE")
    )
private Set<Entity> entities;

EDIT: I forgot to mention that you only need to write this only on one of the entities, the other one doesn't need the @JoinTable annotation!

Answer 2

I would say add fetch = FetchType.Lazy , for the lazy loading which is preferred in most of the scenarios, just like below:

@ManyToMany(cascade = {
        CascadeType.ALL
    }, fetch = FetchType.LAZY)
    @JoinTable(name = "TABLE NAME WHERE RELATIONS ARE STORED",
        joinColumns = @JoinColumn(name = "ID_FIRST_TABLE"),
        inverseJoinColumns = @JoinColumn(name = "ID_SECOND_TABLE")
    )
private Set<Entity> entities;

Answer 3

Every answer here are kinda correct BUT! your object have to be "recursive".

Try this:

User.java

@Data
@AllArgsConstructor
@NoArgsConstructor
@Entity
@ToString(exclude = "roles")
@Table(name = "user")
public class User {

    @Id
    @GeneratedValue
    @Column(name = "user_id")
    private Long id;

    @Column
    private String username;

    @ManyToMany(cascade = { CascadeType.ALL })
    @JoinTable(name = "user_role",
            joinColumns = { @JoinColumn(name = "user_id") },
            inverseJoinColumns = { @JoinColumn(name = "role_id") })
    private List<Role> roles = new ArrayList<>();


    /*
    We need to add methods below to make everything work correctly.
    Use them to add or delete roles!!!
     */

    public void addRole(Role role) {
        roles.add(role);
        role.getUsers().add(this);
    }

    public void removeRole(Role role) {
        roles.remove(role);
        role.getRoles().remove(this);
    }

}

Role.java

@Data
@AllArgsConstructor
@NoArgsConstructor
@Entity
@ToString(exclude = "users")
@Table(name = "role")
public class Role{

    @Id
    @GeneratedValue
    @Column(name = "role_id")
    private Long id;

    @Column
    private String roleName;

    @ManyToMany(mappedBy="roles")
    private List<User> users = new ArrayList<>();

    /*
    We need to add methods below to make everything work correctly
     */

    public void addUser(User user) {
        users.add(user);
        user.getRoles().add(this);
    }

    public void removeUser(User user) {
        users.remove(user);
        user.getRoles().remove(this);
    }

}