Many to many on the same Entity

Question

I want to create a instagram-like application. So user is supposed to have list of users that follows and also a list of users that are following him.
Here's what i've done so far:

@ManyToMany(cascade = {
        CascadeType.PERSIST,
        CascadeType.MERGE
})
@JoinTable(name = "following_followers",
        joinColumns = @JoinColumn(name = "user_id"),
        inverseJoinColumns = @JoinColumn(name = "followedby_id")
)
private List<User> followers;

@ManyToMany(cascade = {
        CascadeType.PERSIST,
        CascadeType.MERGE
})
@JoinTable(name = "following_followers",
        joinColumns = @JoinColumn(name = "followedby_id"),
        inverseJoinColumns = @JoinColumn(name = "user_id")
)
private List<User> following;

I checked this post and I've done it following the good answer over there, but I'm not quite sure does this work.

and you don't say WHAT "doesn't work", nor do you explain why you are reusing a join table for 2 different purposes. If you mean those relations to be opposite ends of a BIDIRECTIONAL relation then you put `mappedBy` on one side. All in JPA docs — , Jan 27 '19 at 18:19

v.ladynev · Answer 1 · 2021-03-04T12:47:28.253

1

When many-to-many association is managed by Hibernate, you can't control the join table following_followers yourself. For example, you can't delete a record from the table using Hibernate.

There are situations for which many-to-many association can be effective, but I am not sure this is your case. For example, imagine how you manage to delete a follower of a user.

You can try another approach: add a separate entity for a following relation

@Enity
class Following {

  @ManyToOne(fetch = FetchType.LAZY)
  User from;

  @ManyToOne(fetch = FetchType.LAZY)
  User to;

}

Also you can add a unique constraint: (from, to) can be suitable.

edited Mar 04 '21 at 12:47

answered Jan 26 '19 at 15:28

v.ladynev

19,275
8
46
67

So your advice would be that I should create another class and a table, where I would make records of from - to relationship? – Strahinja Jan 26 '19 at 15:35
@StrahinjaRodic Yes. And you already have the table `following_followers` anyway. And all operations with followers better to perform using `Following` (not the `User`). Better even don't have `Following` associations (fields) in the `User`. – v.ladynev Jan 28 '19 at 10:32

score 0 · Answer 2 · answered Jan 26 '19 at 21:02

This was my solution for your problem. The user class:

@Entity
public class User {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private long id;

    private String name;

    @OneToMany(mappedBy="to")
    private List<Followers> followers;

    @OneToMany(mappedBy="from")
    private List<Followers> following;

    public User() {}

    public User(String name) {
        this.name = name;
    }

}

As you can see the user class has a reference to the users who is following and those who he follows.

@Entity
public class Followers {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private long id;

    @ManyToOne
    @JoinColumn(name="from_user_fk")
    private User from;

    @ManyToOne
    @JoinColumn(name="to_user_fk")
    private User to;

    public Followers() {};

    public Followers(User from, User to) {
        this.from = from;
        this.to = to;
    }
}

The followers class contain the relationship between the two users, of course you might need to add more information to that class like when 'from' started following 'to'.

If you want to get the followers/following of a user you just call the respective getter.

Better to not use `User` to deal with `Followers`. Even don't provide associations in the `User` to `Followers`. — v.ladynev, Jan 28 '19 at 10:33
I agree with you, you should create a repository for followers with all the related queries like the number of followers, am i following this person and more... But as i understood from the question he wanted something in one class, so this is the solution if you want it in one class. — Muni, Jan 29 '19 at 19:09
@v.ladynev why it's not correct to use User in Followers class? — gmexo, Jan 23 '20 at 10:44
@gmexo My comment was not about `Followers` class, but about `User` class. It is better to not have any mentions of `Followers` in the `User` class. — v.ladynev, Jan 27 '20 at 11:15

Many to many on the same Entity

2 Answers2