Deleting a Many to Many entry while keeping both objects in the database

[Mason Wyatt]

I currently have a Many to Many relationship between Events and Users. The auto generated table in my DB called event_registrations keeps track of the relationship and which user goes to which event based on their ids.

What I want to do is have a controller method that takes in an Event id together with a list of user IDs so that the given users get removed from the given event.

Here are my model classes:

@Entity
public class Event {
  @Id
  @GeneratedValue(strategy = GenerationType.AUTO)
  private Long id;

  @ManyToMany(mappedBy = "eventRegistrations")
  private List<User> userList;

  public Event() { this.userList = new ArrayList<>(); }

  public Long getId() {
      return id;
  }

  public void setId(Long id) {
      this.id = id;
  }

  public List<User> getUserList() {
      return userList;
  }

  public void registerUser(User user){
      this.userList.add(user);
  }
  public void removeUserRegistration(long userId){
      this.userList.removeIf(user -> user.getId() == userId);
  }
}


@Entity
public class User {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Integer id;

@ManyToMany
@JsonIgnore
@JoinTable(
        name = "event_registrations",
        joinColumns = @JoinColumn(name="user_id", referencedColumnName = 
"id"),
        inverseJoinColumns = @JoinColumn(name = "event_id", 
referencedColumnName = "id"))
private List<Event> eventRegistrations;

public Integer getId() {
    return id;
}

public List<Event> getEventRegistrations() {
    return eventRegistrations;
}

public void setEventRegistrations(List<Event> eventRegistrations) {
    this.eventRegistrations = eventRegistrations;
}
}

What I've tried so far in the EventController:

@DeleteMapping(value = "/{id}/registrations", consumes = 
{"application/json"})
public ResponseEntity deleteEventRegistrations(@RequestBody ArrayList<Long> 
data, @PathVariable("id") long id){
    try {
        Event event = eventService.getEventById(id);
        data.forEach(userId -> event.removeUserRegistration(userId));
        return ResponseEntity.ok().build();
    } catch(DataNotFoundException ex){
        return ResponseEntity.notFound().build();
    }
}

This runs without problems but the the entries still exist in the join table afterwards. When debugging this, the users do get deleted from the Event object but the changes don't get persisted to the database.

Any help is appreciated!

[Rob Spoor]

Depending on the JPA implementation, you may have to remove from the owning side. I've had problems after migrating from WebLogic + TopLink to Spring Boot + Hibernate where many-to-many links weren't updated correctly if I updated the non-owning side.

In your case, you have two sides: User is the owning side, and Event is the non-owning side (you can see this from the "mappedBy" on Event). That means that you should remove from User.eventRegistrations, not from Event.userList.

A quick attempt without any additional helper methods:

@DeleteMapping(value = "/{id}/registrations", consumes = 
{"application/json"})
public ResponseEntity<?> deleteEventRegistrations(@RequestBody ArrayList<Long> 
data, @PathVariable("id") long id){
    try {
        Collection<User> users = userService.findUsersByIds(data); // you need to create this
        users.forEach(user -> user.getEventRegistrations().removeIf(event -> event.getId() == id));
        return ResponseEntity.ok().build();
    } catch(DataNotFoundException ex){
        return ResponseEntity.notFound().build();
    }
}