i've been playing with the NHibernate sources to support a mapping we need, and whilst testing it I found that adding children to a parent collection wasn't working. I scaled back to the simple case that should have worked without any mods, but that exhibited the same problem. After getting nowhere looking for the error, I plugged my home built library into an application that worked with the downloaded library, expecting to get the same problem, but it worked. I'm now confused, and suspect I've done something daft with my code or mapping, but I can't see what. Can anyone point me at the doubtless glaringly obvious mistake?

Here are the test tables, pretty simple:


Here is the mapping:


and here's the sample code that's giving the error:


Apart from the properties given, IDParent and IDChild just consist of simple constructors and simple scalar properties for the remain properties in the mappings.

When I run the code, I get the error


If I remove the child.Parent=parent lines, I get the following error instead:


Presumably I've got a mistake somewhere, but so far examing it or comparing it to similar code in the working application hasn't shed any light.

Incidentally, looking at the examples from the documentation, the latter should work, but in my (working) apps I always have to set the parent reference explicitly - which way should it work?

Thanks,
Kev

I think we explicitly call session.Save(child) after each child is added to a parent's collection, even though it shouldn't be necessary. I seem to remember NHibernate having some bug where doing this helped ...

Hi Nels,
I'll give it a try when I get in tomorrow, but I'm not sure it's related. The working application doesn't do this, but works okay when adding multiple children to a collection

Kev

Aha, spotted it - I knew it was something daft. In my working example the classes are more intelligent, and amongst other things automatically set the collection to a new instance if the property get is accessed and the list is not yet set. In my test code I was always setting the list even after a load, and doing so for a loaded object causes the dereference error

Still doesn't work if I just add the child to the parent's collection without setting the reference in the child as well - are the docs just wrong when they say you can do this?

Kev

Yeah, you're right about having to set the collection instance manually if it's null, we have to do the same.

From what I remember, you do have to set the reference to the parent on the child as well as adding the child to the parent's collection. We solved the problem by making our own collection class that knows who the parent ("owner") is and what the collection item type's "owner" property name is. Then when you add (or remove) items to an "owned" collection, it automatically sets (or clears) the "owner" on the item. I don't know why more people don't do this, since we have quite a number of entities with a dozen or more collection properties, and it clutters up the entity's API to have to define methods like AddFoo(), AddBar(), etc. for every collection property (not to mention it stinks of poor OO design).

For child entities that must belong to a parent I tend to make the default constructor private and provide a public constructor that takes a reference to the parent. Your way seems neater though