Problem with class and superclass

N@ruto · **Joined:** Wed Feb 07, 2007 12:48 pm **Posts:** 3

Hi to everybody!
I read some old topic but i don't found nothing useful for this issue.
I want to know if it is really possible to load from db a single object without pull up the entire child class instance.
In other word i need to load the class A that have been specialized in class B, class C.....class Z, i don't want to need to load one istance for each single class, i just want an istance of class A!

I know about the existence of polymorphis tag and annotation but i understand, throw other topic, that this doesn't work in the actual versione of hibernate.

thank to everybody, and i apologise for my terrible english.

N@ruto

emmanuel · **Posted:** Wed Mar 21, 2007 10:59 am

N@ruto wrote:

I know about the existence of polymorphis tag and annotation but i understand, throw other topic, that this doesn't work in the actual versione of hibernate.

I am not aware of a bug, and for sure no one gave me a reproducible test case for it.
For you case, it is not supported in Hibernate because it is not supported in Java. If you have an instance of class B (subclass of A), there is no way (and no need) to convert it into an instance of A only, it is by essense an instance of class B.
What polymorphism=explicit does is

B subclass of A and B polymorphism explicit and B the root entity (not a subclass),
Then from A will not return Bs
More formally

Quote:

Implicit polymorphism means that instances of the class will be returned by a query that names any superclass or implemented interface or the class and that instances of any subclass of the class will be returned by a query that names the class itself. Explicit polymorphism means that class instances will be returned only by queries that explicitly name that class and that queries that name the class will return only instances of subclasses mapped inside this <class> declaration as a <subclass> or <joined-subclass>. For most purposes the default, polymorphism="implicit", is appropriate. Explicit polymorphism is useful when two different classes are mapped to the same table (this allows a "lightweight" class that contains a subset of the table columns).

N@ruto · **Joined:** Wed Feb 07, 2007 12:48 pm **Posts:** 3

I don't want to convert B into an instance of A.
Maybe i wrong to explain.

I have this :

Class A
Class B extend A
Class C extend A

if i try to load a object of class A in the query that make the selection i see that there is a join that involve the table of class A, class B and class C, but this is not usefull if i try to load just A.

I have this problem also i try to load just an object of Class B, i see the same join with the table A (right) and join with table C!!

I try to make a simple test about this (i don't be able to send my application :D) and show you this.

thank you very much for the answer.
N@ruto

emmanuel · **Posted:** Wed Mar 21, 2007 12:33 pm

This is the expected behavior. We don't upcast.