Spring在更新对象时启动多对一映射错误

我正在研究spring boot,并使用多对一关系,如下所示:

@Entity
@Table(name = "media_files")
public class Media {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  @OneToMany(mappedBy = "media", cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval=true)
  private Set<MediaUser> users = new HashSet<>();

  ....

}

使用者

@Entity
@Table(name = "users")
public class User {

  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private Long id;

  @OneToMany(mappedBy = "user", cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval=true)
 private Set<MediaUser> users = new HashSet<>();
 ....
}

媒体用户

@Entity
@Table(name = "media_to_users")
public class MediaUser implements Serializable {

   @Id
   @ManyToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
   @JoinColumn(name = "media_id")
   private Media media;

  @Id
  @ManyToOne(cascade = CascadeType.ALL, fetch = FetchType.LAZY)
  @JoinColumn(name = "user_id")
  private User user;


  @JsonBackReference
  @ManyToOne
  @JoinColumn(name = "parent_id")
  private Media parent;
  .....
 }

我添加了这样的新实体

media.getUsers().add(new MediaUser(media, user, parent));

这很好,但是如果我尝试更新,就会出现错误

//Get media from db
Media media = mediaRepository.getOne(id);
//clear already existing entries
media.getUsers().clear();


   String[]  users = {"3","2"};
    for (String userIdStr: users) {
        User user = userRepository.getOne(Long.parseLong(userIdStr));
        if(user != null){
             media.getUsers().add(new MediaUser(media, user, null));
            //mediaUserSet.add(new MediaUser(media, user, null));
        }
    }

 mediaRepository.save(media);

错误是:

  java.sql.SQLIntegrityConstraintViolationException: Column 'user_id' cannot be null
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:117) ~[mysql-connector-java-8.0.13.jar:8.0.13]
    at com.mysql.cj.jdbc.exceptions.SQLError.createSQLException(SQLError.java:97) ~[mysql-connector-java-8.0.13.jar:8.0.13]