subclass instance from existing superclass

mbatsis · **Joined:** Wed Sep 17, 2003 10:43 am **Posts:** 48

Dear tribe,

I'm trying to figure out how to create an instance of a subclass for an instance of a superclass that already exists.

Consider two subclasses, Human and Woman (a joined-subclass of Human). Now, suppose i have a human record with 1 as the primary key. I tried creating the subclass instance like:

Code:

// get the superclass instance first
Human human = sess.load(Human.class, humanId);

// now create the subclass instance
Woman woman = new Woman();
woman.setSomething(something);

// try to save the woman 
sess.save(woman, human.getId());

but the final line results in

net.sf.hibernate.NonUniqueObjectException: a different object with the same identifier value was already associated with the session

my problem is that i have to create this subclass instance while the superclass instance is already there. Is there a way to do that or should i try to look at alternative ways to map this?

Thanks for your time and help,

Manos

ko5tik · **Posted:** Mon Nov 08, 2004 9:14 am

Why not make superclass abstract?

mbatsis · **Joined:** Wed Sep 17, 2003 10:43 am **Posts:** 48

Hi ko5tik,

Thanks for your reply.

Well, because Human is used in an other part of the application as a concrete class. I later have to use a Human record as an instance of Woman by extending it (add additional properties etc) and keeping any relationships with the original record intact. Essentially, i just want to take advantage of the OO semantics and the joined subclass infastructure to extend a record.

Thanks for any pointers,

Manos

michael · **Posted:** Tue Nov 09, 2004 5:19 am

No, this is not possible. Either delete Human first, use JDBC or rethink your object model. Objects can not just change their type - this has been discussed extensively already in the forum.

mbatsis · **Joined:** Wed Sep 17, 2003 10:43 am **Posts:** 48

Hi Michael,

Thanks for your help and time (you saved a lot of mine ;-)

Essentially, I wanted to extend the functionality of a record for a new component of the application and thought it may be possible; after all, it would just be an FK from the Woman to Human, as it will become now (as a normal one-to-one).

I'm currently in the design process of another application that may require somethng like this; that is, discover at some point in time that a certain instance of Foo, actually is an instance of a subclass of Foo and the additional properties/information is available (to add the joined record). Do you think this is doable/desired? If so i will submit this on Jira and help as i can along the way.

Thanks again,

Manos

michael · **Posted:** Tue Nov 09, 2004 6:38 am

I don't think this will be implemented, as this is not possible either in the obejct-world (you can't just change an objects class). I'd suggest if you design an application that needs such a feature, not to use subclassing but aggregation for it.

tsouza · **Joined:** Sun Feb 08, 2004 4:09 pm **Posts:** 46

In object-world you can't change an object type, althought an object may be specialized sometimes, ie:

Employee
/\
|
|
Supervisor

Where employee is not abstract and supervisor has some overwritten behaviors, so, an employee may become a supervisor...

Cya!

michael · **Posted:** Thu Nov 25, 2004 12:46 pm

I'd really like to see how you do that:

Object o = new Supervisor();

... do some magic

(o instanceof Supervisor) == false;

tsouza · **Joined:** Sun Feb 08, 2004 4:09 pm **Posts:** 46

Well,

Instantiate new Supervisor, copy Emplyee's information, fill-in with aditional information that supervisor may need, keep same id, persist the new object...

michael · **Posted:** Thu Nov 25, 2004 9:25 pm

Thats not the same object! you can do just that with hibernate: instantiate new object, copy things over, delete old one, persist new one.

tsouza · **Joined:** Sun Feb 08, 2004 4:09 pm **Posts:** 46

Hmm, yeh, i understand what you're saying...