Hibernate自引用关系外键约束问题

Question

I am setting up a Springboot API which uses hibernate to communicate with the database. I have a User table, and each user should be able to follow other users. I am not experienced with hibernate, so after a while I got something that seemed to work, except that I cannot delete a user if that specific user is being followed by other user(s). This is because of a foreign key constraint.

This is what the user looks like:

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(unique = true)
    @JsonProperty
    private int id;

    @Column(unique = true)
    @JsonProperty
    private String email;

    @Column
    @JsonProperty
    private String hashedSaltedPassword;

    @Column
    @JsonProperty
    private String salt;

    @Column
    @JsonProperty
    private String firstName;

    @Column
    @JsonProperty
    private String lastName;

    @JsonProperty
    @ManyToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JoinTable(name = "User_Follows", schema = "todo", joinColumns=@JoinColumn(name="user_id"))
    private List<User> follows;

This creates the user table and a seperate table called User_Follows. This table has the user_id and a follows_id, which is the userId that is being followed. This is pretty much what I want.

Deleting a user works perfectly as long as that user isn't being followed by other users (so as long as the userId of that user isn't in the follows_id column).

But it gives the following error if it is:

The DELETE statement conflicted with the REFERENCE constraint "FKc38v25qxktm9b0r7lg1s2t1tl". The conflict occurred in database "dbi390100_db", table "todo.User_Follows", column 'follows_id'.

This should not happen, what I want is that all rows where the userId exists of the User_Follows table will be deleted whenever the user is being deleted.

Is there a solution for this?

Answer 1

There are two ways of accomplishing what you want.

• Define a mappedBy property with another field to let hibernate know about the entire relationship, something like:

@ManyToMany(targetEntity = User.class, mappedBy = "follows")
private List<User> followers = new ArrayList<>();

You can have a look at this other question

• Or you can create a @PreRemove method in your user class to manually delete all references of your User:

@PreRemove
private void removeFromFollowers() {
   for (User user : followers) {
       user.removeFollowed(this);
   }
}