Question

As per the title, I have an object which has a ReferencesAny association, but it’s possible that the referenced object may have been deleted. If this has occurred, currently the missing object is initialized with a proxy which then throws ObjectNotFound if accessed. I want to be able to have this return null instead, in a similar fashion to setting not-found to “ignore” on other properties. I understand the differences between using Session.Get and Session.Load but this is occurring as a result of a LINQ query.

So, my question is, how can I specify that null should be returned for missing objects for an “any” association?

Answer 1

I am using xml configuration, but I guess that my hint could help you anyway… NHibernate provides for those incomplete mappings an attribute not-found which is by default set to “exception“. In this example is reference many-to-one mapping, but it could be used even for one-to-many

<many-to-one name="MyReferencedObject" not-found="exception" />

but it can be changed to ignore

<many-to-one name="MyReferencedObject" not-found="ignore" />

In that case the result will be NULL. (But be careful, NHibernate will always execute a SQL select statement, because such a “NULL” value won’t be cached)

EDIT: based on the comment below, I have to say, that my suggestion is not working for <any> mapping. Sorry for that, I should read more carefully…
But, please, let me provide you with suggestion, how to fix it.

We can introduce event listener. More can be found here http://nhibernate.info/doc/nh/en/index.html#objectstate-events

Solution in a nutshell:

Introduce the event listener

public class PostLoadListener 
           : NHibernate.Event.Default.DefaultPostLoadEventListener
{ ... }

and inject it into the “session-factory” configuration.

<event type="post-load">
    <listener class="MyLib.PostLoadListener, MyLib"/>
</event>

The trick would be inside of the overriden method OnPostLoad

public override void OnPostLoad(PostLoadEvent @event)
{
    base.OnPostLoad(@event);

    // the entity with <any> mapping 
    ConvertToNull(@event.Entity as MyAuditEntity); 
}

protected virtual void ConvertToNull(MyAuditEntity item)
{
    if (item == null)
    {
        return;
    }
    try
    {
        // access some property to check that reference is not a PROXY
        var id = item.AnyEntity.ID;
    }
    catch
    {
        // replace proxy with null
        item.AnyEntity = null; 
    }
}

Of course, this is not the answer like not-found=”ignore”. But this workaround will do that job.