java - 删除多对多条目，同时将两个对象都保留在数据库中

Question

我目前在事件和用户之间存在多对多关系。我的数据库中自动生成的名为 event_registrations 的表会跟踪关系以及哪个用户根据他们的 ID 去哪个事件。

我想要做的是有一个 Controller 方法，它接收事件 ID 和用户 ID 列表，以便从给定事件中删除给定用户。

这是我的模型类:

@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;
}
}

到目前为止，我在 EventController 中尝试过的内容:

@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();
    }
}

这运行没有问题，但之后条目仍然存在于连接表中。对此进行调试时，用户确实会从 Event 对象中删除，但更改不会持久保存到数据库中。

感谢任何帮助!

Answer 1

@Entity
public class Event {

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

}

这里的mappedBy是指User的eventRegistrations列表用来维护这个多对多的关系，也就是说Hibernate会更新基于 User 的 eventRegistrations 列表内容的关系表 (event_registrations)。你必须反过来做，从用户的事件列表中删除该事件:

public void removeUserRegistration(long userId){
     //remove the event from the given user 's event list 
     for(User user : userList){
          if(user.getId().equals(userId)){
               user.getEventRegistrations().removeIf(event->event.getId().equals(this.id));
           }
     }

     //remove given user from the event 's user list 
     //This will not have effects on DB record (as mentioned above) but suggest to also do it for keep the data in java model to be consistency.
     this.userList.removeIf(user -> user.getId() == userId);
 } 

以上代码只是为了展示思路。您可能需要对其进行抛光。