JPA ManytoOne和OneToMany关系不持久

Question

I have two entities, a Competition and a Team:

Competition:

@Entity
public class Team implements Serializable{
private static final long serialVersionUID = 1L;
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
int id;

@Column(length=255)
String name;

@ManyToOne(optional=true)
Competition nextCompetitionAttending;
List<SiteUser> teamMembers;

nextComeptitionAttending is set by the following method:

public void setNextCompetitionAttending(Competition nextCompetitionAttending) {
    this.nextCompetitionAttending = nextCompetitionAttending;
    nextCompetitionAttending.addTeam(this);
}

Competition is defined thusly:

@Entity
public class Competition implements Serializable{

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
int id;
int maxTeamSize;
String name;

@OneToMany(mappedBy="nextCompetitionAttending" )
List<Team> teamsAttending;

My persistance code looks like this:

             utx.begin();
        EntityManager em = emf.createEntityManager();
        em.merge(user);
        Competition c = em.find(Competition.class, id);
        Team teamon = user.getTeamOn();
        teamon.setNextCompetitionAttending(c);
        System.out.println("Set next");
        em.flush();
        utx.commit();
        em = emf.createEntityManager();
        Team t = em.find(Team.class, teamon.getId());
        System.out.println(t.getNextCompetitionAttending());

In the database, the column "NEXTCOMPETITIONATTENDING_ID" is null, as is the println at the end of the code which I've been using to try to debug this.

Why doesn't this persist?

Answer 1

The culprit is probably this line:

em.merge(user);

user is a detached entity, and you merge it. Then, you get the team from the user. But merge doesn't make the detached entity attached. It loads the attached user, copy the state from the detached one to the attached one, and returns the attached one. So, all the subsequent changes you're doing are done to the detached entity.

Replace this line with

user = em.merge(user);