NHibernate: "collection was not processed by flush()" caused by lazy loading issue

Question

I have two classes:

class Parent
{
    public virtual Child Child { get; set; }
}

class Child 
{
    public virtual IList<GrandChild> GrandChildren { get; set; }
}

I have an instance of Parent loaded from my ISession, Parent.Child is lazy loaded (NOT loaded at this point). Child.GrandChildren is also lazy loaded.

If I do this:

session.Save(new Parent { Child = existingParent.Child } );

I get collection [Child.GrandChildren] was not processed by flush()

If I cause existingParent's Child property to be loaded, simply by accessing it:

var x = existingParent.Child.Name

the problem goes away. Why is this happening, and how do I solve it - preferably without having to change my fetching strategy?

*Edit: * Parent has a FK to Child

I'm using NH 2.1.2.4000

Thanks

Answer 1

What is the cascade setting for cascading changes from Child to the GrandChildren collection? I think NHibernate throws this exception if the collection is dirty but the cascade setting does not cause the changes to be persisted.

Answer 2

You can use session.Load to reference an existing instance of Child without making a trip to the db. This should do it, I think:

session.Save(new Parent { Child = session.Load(existingParent.Child.Id) } );

But check to make sure that the .Id call doesn't trigger a db trip.