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gamigin
 Post subject: Inheritance Problem with the "table per subclass"
PostPosted: Fri Oct 07, 2005 9:59 am 
Newbie

Joined: Thu Oct 06, 2005 8:08 pm
Posts: 19
Inheritance Problem with the "table per subclass" aka "joined-subclass" model

My HQL query is returning instances of the base class rather than the correct derived class. I was wondering if this is normal or if I am doing something wrong. Please help!

Simplified class structure:

Code:
class A
{
   private int id;
   private B b;   // Reference to class b

   // trivial getters/settters
};

class B
{
   private int id;
   private String someData;

   // trivial getters/settters
};

class C extends B
{
   private String derivedClassData;

   // trivial getters/settters
};


C.hbm.xml:

Code:
<joined-subclass
      name="package.C"
      table="C"
      extends="package.B">
   <key column="ID"/>

   <property name="derivedClassData" column="DerivedClassData" type="string" not-null="true"/>
</joined-subclass>


My simplified code:

Code:
   Session session = HibernateUtil.getCurrentThreadSession();   
   try {   
      Query query = session.createQuery("from A as a");
      searchResults = query.list();

      for (Object o : searchResults) {
         A a = (A) a;
         B b = a.getB();

         // This will throw ClassCastException. Hibernate always returns direct instances of B
         // and never detects and creates the derived instances of C. Is there any way to get it
         // to do that? For this problem, trust me that every B instance will be a direct instance of C.
         C c = (C) b;
      }
   } finally {
      HibernateUtil.closeSession();
   }
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steckemetz
 Post subject:
PostPosted: Fri Oct 07, 2005 11:27 am 
Pro
Pro

Joined: Mon Jan 24, 2005 5:39 am
Posts: 216
Location: Germany
Interesting.

Did you try

Code:
Query query = session.createQuery("from B as b");

or
Code:
Query query = session.createQuery("from C as c");


?
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famousactress
 Post subject:
PostPosted: Fri Oct 07, 2005 11:45 am 
Beginner
Beginner

Joined: Mon Sep 27, 2004 4:28 pm
Posts: 44
Hibernate is really good about keeping us honest with Polymorphism. If you have a relationship with the base class, it assumes that's what you want. If you know it's an instance of C, then you can certainly do the following:

C c = (C) session.get(C.class, b.getId());

Hibernate is building you a collection of B objects, but avoiding the potentially numerous queries it would take to gather all of the subclass data. Whenever I run into this problem with Hibernate, it occurs to me that I'm probably using polymorphism poorly... For instance, if your class will always be a 'C'.. then maybe you should give A a relationship to C instead?

I believe you can also make this behavior go away by disabling the proxying of your inherited classes... This way hibernate can't create a base-class proxy, and will instead need to construct real instances of your classes.

Hope this is helpful, it's been a while since I've run into this so there's a chance I'm misspeaking :)

Phill
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gamigin
 Post subject:
PostPosted: Fri Oct 07, 2005 12:30 pm 
Newbie

Joined: Thu Oct 06, 2005 8:08 pm
Posts: 19
famousactress wrote:
For instance, if your class will always be a 'C'.. then maybe you should give A a relationship to C instead?


With the simplified scenario I presented that would be a good idea. In fact, I really don't need inheritance at all if this was my end goal; however, in the near future I will need inheritance and A will need to refer to derived classes of B other than C, so I would like to get this class model working.

Thanks guys. You've both given me two ideas to try:

- "from B as b". I know "from C as c" works but this will test that inheritance is setup correctly.
- Setting class B to abstract. It doesn't have any abstract members but conceptually it should be abstract since there should never be a direct instance of B. This might help Hibernate load the proper instances.

I will try these tonight and post the results. Thanks!
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famousactress
 Post subject:
PostPosted: Fri Oct 07, 2005 12:37 pm 
Beginner
Beginner

Joined: Mon Sep 27, 2004 4:28 pm
Posts: 44
Beware that setting B to abstract might not solve your problem. Hibernate via CGLib will still be able to create proxy classes of B and return them to you. I believe the real way to stop this from happening is by setting lazy="false" at the class level of your hbm.xml mapping.

Oh, and you're welcome.. feel free to press the "Yes, this helped me button on our posts to give us credits :) "

Good luck,
Phill
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