Question

@Entity
public class Group
{
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Key id;
}


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

    @ManyToOne(fetch = FetchType.LAZY)
    private Group group;
}

After the following code :

EntityManager entityManager = EntityManagerFactoryHolder.getEntityManagerFactory().createEntityManager();
Group group = new Group();
entityManager.persist(group);
User user = new User();
user.setGroup(group);
entityManager.persist(user);
entityManager.close();

I get the following error

Detected attempt to establish User(28) as the parent of Group(27) but
the entity identified by Group(27) has already been persisted without
a parent. A parent cannot be established or changed once an object
has been persisted.
org.datanucleus.store.appengine.DatastoreRelationFieldManager$ChildWithoutParentException:
Detected attempt to establish User(28) as the parent of Group(27) but
the entity identified by Group(27) has already been persisted without
a parent. A parent cannot be established or changed once an object
has been persisted.

Answer 1

Seems that this only works with back association. Added @OneToMany set to Group object and it works now.