[hibernate-issues] [Hibernate-JIRA] Created: (HHH-2477) lazy fetching ManyToOne produces inproper proxies when using single table inheritance strategy

[Daniel Nguyen (JIRA)]

lazy fetching ManyToOne produces inproper proxies when using single table inheritance strategy
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         Key: HHH-2477
         URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-2477
     Project: Hibernate3
        Type: Bug

  Components: core  
    Versions: 3.2.1    
 Environment: hibernate 3.2.1-GA, postgresql 8.1
    Reporter: Daniel Nguyen
 Attachments: bug.zip

I have entity base class Vehicle and entity class Car which extends Vehicle (using single table inheritance strategy).
I also have entity class User which have single-ended @ManyToOne relation from User to Vehicle with lazy fetching.

Now there is a Car instance "c" and User instance "u" with relation to "c". Lazily fetching vehicle from "u" results with proxy object "o" which is instanceof Vehicle but not instanceof Car as expected. So it's impossible cast "o" to Car or at least read any of Car's property from "o".

This is serious problem when inheritance is extensively used because inproper proxy object remains in cache. To walkaround I'd have to resign with lazy fetching (much performance loss) or manually replace proxy object in cache.

testing code:

    SessionFactory sf = null;
    Session s = null;
    Transaction tx = null;
    try
    {
      sf = cfg.buildSessionFactory();
      s = sf.openSession();
      tx = s.beginTransaction();
      
      //create car
      Car car = new Car();
      car.setIdVehicle(1);
      car.setAge(5);
      s.save(car);
      
      //create user of car
      User user = new User();
      user.setIdUser(1);
      user.setVehicle(car);
      s.save(user);
      
      //make sure it is actually added
      s.flush();

      // test 1 - works ok, because we loaded vehicle before user
      s.clear(); //clear cache
      Vehicle vv = (Vehicle)s.get(Vehicle.class, 1);
      User uu = (User)s.get(User.class, 1);
      //we know that user vehicle is actually a Car
      Car cc = (Car)uu.getVehicle();
      
      // test 2 - fails, when using lazy fetching
      s.clear(); //clear cache
      User u = (User)s.get(User.class, 1);
      Vehicle v = u.getVehicle();
      //we know that user vehicle is actually a Car
      //so we cast to Car but ClassCastException is raised!!!
      Car c = (Car)v;
      //let's check what's actual class of v
      System.out.println(v.getClass().getName());
      System.out.println(v.getClass().getSuperclass().getName());
      //result:
      //  test.hibernate.Vehicle$$EnhancerByCGLIB$$12ce1883
      //  test.hibernate.Vehicle
      //but I expected:
      //  test.hibernate.Car$$EnhancerByCGLIB$$xxxxx
      //  test.hibernate.Car
    }
    finally
    {
      if (tx!=null)
        tx.rollback();
      if (s!=null)
        s.close();
      if (sf!=null)
        sf.close();
    }


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