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

[S.Schnabl (JIRA)]

    [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-2477?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_27222 ] 

S.Schnabl commented on HHH-2477:
--------------------------------

The HIA Book states the following for lazy *toOne associations:

"...Hibernate would proxy the lazy association. In this case, we wouldn't be able to perform a typecast to the concrete class CreditCard at runtime, and even the instanceof operator would behave strangely....".

This means, you cannot lazy load a *toOne association and expect, that hibernate knows the real concrete class of your proxy object without hitting the database once. Without a database hit hibernate simply cannot determine the concrete class. To get the concrete class, YOU simply have to know the class and must reload the association from the database, ex. :

Car car = (Car) session.load( Car.class, user.getVehicle.getId() );


> lazy fetching ManyToOne produces inproper proxies when using single table inheritance strategy
> ----------------------------------------------------------------------------------------------
>
>                 Key: HHH-2477
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-2477
>             Project: Hibernate3
>          Issue Type: Bug
>          Components: core
>    Affects Versions: 3.2.1
>         Environment: hibernate 3.2.1-GA, postgresql 8.1
>            Reporter: Daniel Nguyen
>         Attachments: bug.zip
>
>   Original Estimate: 1 hour
>  Remaining Estimate: 1 hour
>
> 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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