I have a parent class Person which maps table Person(pid). I have a subclass called User which maps table User(pid, username). Their mapping is 'joined-subclass'. Now, i have one record in Person, say pid=2. I want to insert a new record User(2,'test').

however, if I do like this:

Person p = session.load(Person.class, pid);
User u = new User();
u.setPid(p.getPid());
u.setUsername("test");
session.save(u);

I got this error:
org.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session:

if i do session.merge(u), i got this error:
org.hibernate.WrongClassException: Object with id: null was not of the specified subclass:


Any idea?

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According to your client code, I am not sure polymorphism is the way to go in this case.

Person and User, even though in Java they might be subclasses, are being treated as concrete subclasses in your Java example.

You can either treat them as a class superclass relationship (possibly with Person Being abstract, or, in your case, with a "table-per-class hierarchy" or "table-per-concrete-subclass" mapping with Person being a concrete class)

or

treat them as a Parent-child relationship, with person "having a User" and taking the id from him, or vice versa, mapped with a "generator foreign".

But you cannot mix both strategies, as presented in your post.