Loading inheritence classes

adcworks · **Joined:** Fri Jun 18, 2004 10:17 am **Posts:** 140

Hi Guys,

I have a parent object A and a child B extending A. I have managed to get Hibernate to persist this inheritence hierarchy to my DB using a joined-subclass configuration.

I have been able to save an object of type B with a method like this

Code:

   public void saveSchedule(A a) {

      try {
         Session session = HibernateUtil.currentSession();
         Transaction tx = session.beginTransaction();
         
         session.save(a);
         
         tx.commit();
         HibernateUtil.closeSession();
         
      } catch (HibernateException hE) {
         logger.error(hE + " " + hE.getMessage());
      }
   }

This works even when the parameter a is of class B.

I now want to load a class out of the database but I want to keep it general. I came up with this...

Code:

   public void loadSchedule(A a, int id) {
      
      try {
         Session session = HibernateUtil.currentSession();
         Transaction tx = session.beginTransaction();

         a= (A) session.load(schedule.getClass(), new Integer(id));

         tx.commit();
         
      } catch (HibernateException hE) {
         logger.error(hE.getMessage());
      }
   }   

I also tried using A.class for schedule.getClass but it always returns just an A and not a B even if the parameter A a is of class B.

I hope this makes sense and you see what I am trying to achieve. I do not want to have to write methods loadScheduleA and loadScheduleB

Thanks! ADC

adcworks · **Joined:** Fri Jun 18, 2004 10:17 am **Posts:** 140

Hm, OK, what I was doing was OK, the problem is that I have 2 calls to my logic object and I am getting an error;

ScheduleLogic.loadSchedule(Unknown Source) : a different object with the same identifier value was already associated with the session: 2, of class:

I have 2 test calls to 2 separate methods in my logic object

Code:

request.setAttribute("aSchedule", logic.loadSchedule(2));

and

Code:

Schedule schedule2 = new DataAcquisitionSchedule();
logic.loadSchedule(schedule2, 2);

I copy the code for these methods below. As far as I can see, they are 2 totally separate calls to methods so why is this causing the error? Is it that the currentSession is already in use when the second call goes in? If I change the first method call to 1 both statements work.

I would probably never need to load the same object sequentially like this, but I would like to understand what is happening here as it seems to me that this cannot be right - that is, surely a sequence of operations on 1 row is quite common??

Code:

   /**
    * Retrive a Schedule from ID.
    *
    */
   public Schedule loadSchedule(int id) {

      Schedule schedule = null;
      
      try {
         Session session = HibernateUtil.currentSession();
         Transaction tx = session.beginTransaction();

         schedule = (Schedule) session.load(Schedule.class, new Integer(id));

         tx.commit();
         
      } catch (HibernateException hE) {
         logger.error(hE.getMessage());
      }
      
      return schedule;
   }   
   
   /**
    * Retrive a Schedule into a passed Schedule.
    *
    */
   public void loadSchedule(Schedule schedule, int id) {
      
      try {
         Session session = HibernateUtil.currentSession();
         Transaction tx = session.beginTransaction();

         session.load(schedule, new Integer(id));

         tx.commit();
         
      } catch (HibernateException hE) {
         logger.error(hE.getMessage());
      }
   }   

Thanks guys, ADC.

emmanuel · **Posted:** Mon Jun 28, 2004 12:50 pm

You can not have 2 instances representing the same row, so using
load(Object, Serializable) will try to populate the Object instance. If one is already loaded by the session, an exception will be thrown

adcworks · **Joined:** Fri Jun 18, 2004 10:17 am **Posts:** 140

Hi,

Thanks, but I do not entirely understand. In my test I call 2 separate methods one after the other. Each method attempts to load the object with ID 2 in a different way. However, I get this error.

The reason I do not understand what you are saying is that when method 1 has finished loading the object, the current session is no longer used.

From what I understand of what you have said, in a web application I can get object 2 loaded and put into the request object for a JSP to render somehow, but then that object is lost to me, but if I try to then reload it again, Hibernate will complain that it was already loaded. Do I therefore have to make sure I always keep instances loaded by Hibernate somewhere so that I do not have to reload them?

Or should I stop using HibernateUtil so that I create a new session for every method I have for loading?

Thanks and sorry for the confusion.

emmanuel · **Posted:** Tue Jun 29, 2004 8:24 am

I may be wrong,. You probably do something like that

load A with session1
close session1

load B with session2
somehow (cascade etc) you load A' (ie same id as A) in session2
you reattach A to session2, since A.id==A'.id, hibernate conplains

adcworks · **Joined:** Fri Jun 18, 2004 10:17 am **Posts:** 140

Here is what I do exactly. My Servlet does the following:

Code:

request.setAttribute("aSchedule1", logic.loadSchedule(2));
Schedule schedule2 = new DataAcquisitionSchedule();
logic.loadSchedule(schedule2, 2);
request.setAttribute("aSchedule2", schedule2);

The calls are

Code:

   /**
    * Retrive a Schedule from ID.
    *
    */
   public Schedule loadSchedule(int id) {

      Schedule schedule = null;
      
      try {
         Session session = HibernateUtil.currentSession();
         Transaction tx = session.beginTransaction();

         schedule = (Schedule) session.load(Schedule.class, new Integer(id));

         tx.commit();
         
      } catch (HibernateException hE) {
         logger.error(hE.getMessage());
      }
      
      return schedule;
   }   
   
   /**
    * Retrive a Schedule into a passed Schedule.
    *
    */
   public void loadSchedule(Schedule schedule, int id) {
      
      try {
         Session session = HibernateUtil.currentSession();
         Transaction tx = session.beginTransaction();

         session.load(schedule, new Integer(id));

         tx.commit();
         
      } catch (HibernateException hE) {
         logger.error(hE.getMessage());
      }
   }   

That's it. I do not see what is wrong with loading Schedule with ID=2 twice in different sessions.

emmanuel · **Posted:** Tue Jun 29, 2004 8:41 am

You load it twice in the same session for sure. And the second load fail