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Problem while loading an object with a lazy loading set

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yannraoul

Post subject: Problem while loading an object with a lazy loading set

Posted: Fri Dec 05, 2003 1:00 pm

Beginner

Joined: Tue Oct 07, 2003 11:36 am
Posts: 46
Location: Rennes, France

Hi,

I use a persistent object A which has a set declared with lazy initialization.
It works perfectly usually, but there is one bad case:

* i create a new object A (new) without setting the Set values
* i save it
* i forward to another servlet action
* this action loads the objet
* the returned object is exactly the one i saved, but the Set of objects is "null", so I can't iterate on it and trig the lazy loading operations

What did I miss ?

Yann

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dimas

Post subject:

Posted: Fri Dec 05, 2003 1:34 pm

Expert

Joined: Tue Sep 16, 2003 4:06 pm
Posts: 318
Location: St. Petersburg, Russia

Are you sure you are not retrieving Set from the object you just created? Because objects loaded by Hibernate from database have non-null Sets even if these Sets are lazy.

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yannraoul

Post subject:

Posted: Fri Dec 05, 2003 1:39 pm

Beginner

Joined: Tue Oct 07, 2003 11:36 am
Posts: 46
Location: Rennes, France

In the doLoad method of SessionImpl, there is :

Code:

      // LOOK FOR LOADED OBJECT
      Object old = getEntity(key);
      if (old!=null) {  //if this object was already loaded
         Status status = getEntry(old).status;
         if ( checkDeleted && ( status==DELETED || status==GONE ) ) {
            throw new ObjectDeletedException("The object with that id was deleted", id);
         }
         lock(old, lockMode);
         if ( log.isTraceEnabled() ) log.trace( "resolved object in session cache " + MessageHelper.infoString(persister, id) );
         return old;
      }

And I get a non-null old object. It seems Hibernate is keeping a reference on my newly saved object but it resend it without setting the Set which need to be lazy-loaded !!!

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dimas

Post subject:

Posted: Fri Dec 05, 2003 1:44 pm

Expert

Joined: Tue Sep 16, 2003 4:06 pm
Posts: 318
Location: St. Petersburg, Russia

Ah... It may be possible if you save object and load it within the same session. Since I have never done that way (usually object is always loaded from another session), I have never expirienced this.

But what lazy collection initialization are you talking about then? :) If collection is null on newly created object, there is nothing to initialize. Can't you just add null check before iteration?

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dimas

Post subject:

Posted: Fri Dec 05, 2003 2:02 pm

Expert

Joined: Tue Sep 16, 2003 4:06 pm
Posts: 318
Location: St. Petersburg, Russia

... or implement your Set getters as:

Code:

public Set getClients()
{
    if (clients == null)
        clients = new HashSet();
    return clients;
}

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yannraoul

Post subject:

Posted: Mon Dec 08, 2003 8:50 am

Beginner

Joined: Tue Oct 07, 2003 11:36 am
Posts: 46
Location: Rennes, France

This would be a way, but I doubt Hibernate simply instantiate a HashSet to perform lazy loading operations. I suppose it is a very special Set which manages the initialization.

I would like to have the opinion of the Hibernate team about my problem...

And I can give more details if needed.

Regards,
Yann

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gavin

Post subject:

Posted: Mon Dec 08, 2003 8:57 am

Hibernate Team

Joined: Tue Aug 26, 2003 12:50 pm
Posts: 5130
Location: Melbourne, Australia

I personally think that dimas suggestion is perfect. Hibernate wraps any HashSet automatically at flush-time.

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yannraoul

Post subject:

Posted: Mon Dec 08, 2003 9:26 am

Beginner

Joined: Tue Oct 07, 2003 11:36 am
Posts: 46
Location: Rennes, France

I tested it, but, as I thought, the used Set is the one I have initialized (new HashSet() )...and it doesn't perform any lazy operations....so I still don't get my data in the set.

:-(

Isn't there a problem in the temp cache of objects ?
When Hibernate temporarily stores in its session the object, couldn't it set the lazy-loadable Sets to a state "NOT_YET_LOADED" or something like that ?

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gavin

Post subject:

Posted: Mon Dec 08, 2003 9:29 am

Hibernate Team

Joined: Tue Aug 26, 2003 12:50 pm
Posts: 5130
Location: Melbourne, Australia

Oh, I understand what you are trying to do.

Don't do this.

Use an object obtained from session.get() or session.load(), never one instantiated with new, if you want to represent an existing row..

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dimas

Post subject:

Posted: Mon Dec 08, 2003 9:30 am

Expert

Joined: Tue Sep 16, 2003 4:06 pm
Posts: 318
Location: St. Petersburg, Russia

What do you expect will Hibernate lazy-load if you JUST CREATED the object?

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yannraoul

Post subject:

Posted: Mon Dec 08, 2003 10:26 am

Beginner

Joined: Tue Oct 07, 2003 11:36 am
Posts: 46
Location: Rennes, France

You're both right. I misunderstood the developer's problem. It was not a newly created object but an object already persisted in the DB...
So, I asked him to do a session.load() instead of NEW Object, and it works !

Thank you dimas and gavin !

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