We use a collection of images stored on the database on the one of the objects in the application. The main object (appraisal) is "alive" during all the application, when the user navigates to the form where the images are shown we initialize the images collections before doing any data binding. We would like to be able to dehydrate this collections when the user navigates away as this is a pretty heavy collection and the performance downgrade is noticeable.

I'm fairly new to NHibernate and any help/comment is more than welcome.






Hibernate version: 2.2

Mapping documents:
<class name="DomainModel.Appraisal, DomainModel" table="Appraisal">
<id name="Id" column="Id" type="Int32" access="nosetter.camelcase-underscore" unsaved-value="-1">
<generator class="identity"/>
</id>


...


<set name="Images" table="Image" lazy="true" access="nosetter.camelcase-underscore" inverse="true" cascade="all-delete-orphan" order-by="PrintOrder">
<key column="Id"/>
<one-to-many class="DomainModel.Image, Chubb.Appraisal.DomainModel" />
</set>




Code between sessionFactory.openSession() and session.close():
NHibernateUtil.Initialize(appraisal);
NHibernateUtil.Initialize(appraisal.Images);


Full stack trace of any exception that occurs:

Name and version of the database you are using:
SQL Server 2005 9.0.3224

The generated SQL (show_sql=true):

Debug level Hibernate log excerpt:


Problems with Session and transaction handling?
No
Read this: http://hibernate.org/42.html

If I'm understanding what you're asking, you've got an object that contains a collection of images, and that collection is initialized only when required.

The parent object is "alive" all the time? Are you running a web application or a forms based application?

Either way, you should be able to call
session.Evict(object obj);

That should release the object from the first level cache (Your session). At that point, you should have a clean collection.

It's a windows forms application. I will try to evict the collection then and post my results back.
Yes, the parent object stays "alive" through out the life of the application.

It was my understanding that evicting did not mark the object for GC, it only made the object transient (non-persistent)...

After testing, it appears that you're correct, evict only marks it as transient. Because of the way NHibernate proxies the classes and collections, I believe your only alternative would be to reload the parent object from the NHibernate Session when they navigate away.

Or...You might be able to to load two different parent objects from the session. Like so:

ParentType A = session.Load<ParentType>(id);

//Stuff

session.flush(); //persist any changes to A
ParentType B = session.Load<ParentType>(id);
B.ImageCollection.Stuff();
session.Evict(B);
B = null;


It's a huge hack, but other than that, I have no clue how to mark a collection to be re-lazy loaded.

Thanks for the reply.
I'm playing with evicting the collection then clearing it before the end user navigates away. I initialize this collection any time the user opens this form. Let's see how it goes, I'll post the results back.

I'm using the following code:


session = NHibernateHelper.GetSession(NHibernateHelper.SessionKey.MyView);
session.Evict(parentObj.MyCollection); //check if this unloads the collection
parentObj.MyCollection.Clear();


However later on when I try to save or update the parentObj I see Nhibernate tries to do the deletions for parentObj.MyCollection