JPA Add or update object - jpa

Question

I'm trying to save an object called Stat using EclipseLink. If the id exists in the db, update that object. If not, create a new object. Here is my object:
#Entity
public class Stat {
#Column
#Id
#GeneratedValue(strategy = GenerationType.AUTO)
public long id;
#JoinColumn
#ManyToOne
public User user;
#Column
public boolean accepted;
#Column
public boolean finished;
public Stat() {
}
}
And here is my method for adding/updating the object.
public long addReplaceStat(Stat stat) {
em.getTransaction().begin();
Stat oldStat = em.find(Stat.class, stat.id);
if (oldStat == null)
em.persist(stat);
else
em.merge(stat);
em.getTransaction().commit();
em.getTransaction().begin();
Stat newStat = em.find(Stat.class, stat.id);
if (newStat != null)
em.refresh(newStat);
em.getTransaction().commit();
return stat.id;
}
My problem is that I get exception at em.refresh() saying "The attribute [id] of class [User] is mapped to a primary key column in the database. Updates are not allowed." And I don't understand why. Shouldn't refresh just update the values of my managed object without problem?
What is the best idiom to do what I want (add or update)?

Answer 1

The JPA spec says:
The semantics of the merge operation applied to an entity X are as
follows:
If X is a detached entity, the state of X is copied onto a pre-existing managed entity instance X' of the same identity or a new
managed copy X' of X is created.
If X is a new entity instance, a new managed entity instance X' is created and the state of X is copied into the new managed entity
instance X'.
[...]
So to add if not existing or update is it exists, you just need to do:
Stat attachedStat = em.merge(stat);
Regarding your problem with refresh, I don't know why it happens. But why are you refreshing an entity that you just loaded from the database? There is nothing to refresh.