Hi,

I have two classes : Person and Student.
A Student is a Person, so I map this using a subclass in a "table-per-hierarchy" strategy (this means using a discriminator-value which identifiates the class).
In the class Student there is a constructor which allows a Person object parameter, so that you can create a Student from a Person (in fact the constructor does a copy of all properties between both objects).
Nevertheless, when I'm trying to create and saving a Student from an existing Person like this :




it creates a new Student row in the database, owed to the fact that I'm not using identies in my classes, but letting hibernate generate them.
But that's not what I want ! I would like that it only updates the discriminator column, to change the object type !
Even if I write an identity property for my classes, I got an error saying that it was not able to save the Student object because there was another object with the same ID in the database (typically the Person object)).
Do you see what I mean ?
Any help would be appreciated...

The Student class is derived from the Person class, right? Hibernate certainly thinks it is, because that's how you've mapped it. So after you've constructed your in-memory Student object, you now have two different objects representing the same row in the table. More importantly, you have an object in Hibernate's session cache (the Person object) that represents the same thing as an object not in Hibernate's cache (the Student object). When you go to save the Student, this conflict is detected, and then much barfing occurs.

Once you've created your Student object, you must get rid of your Person object. Evict it, or just clear the entire session. So long as you've copied the id property from the Person object to the Student object, Hibernate will know that the Student object is to be saved to the same row as the old Person object: the only thing that will change in the DB row is the value in the discriminator column.

Also destroy the Person object. Do this after you've evicted it, so that Hibernate doesn't try to delete the DB row.

Thank you very much !

If You want to avoid troubles, redesign Your system so that You do not need to change the type of objects. Like using Person-PersonRole pattern.

Gert