NSArrayController predicates with Core Data Relationships

Hi,
I've spent quite a bit of time on this problem today
The Background:
I've got a Core Data model that contains two entities. The first is a container, that has a To-Many relationship called containerItems. The second is a containee that has the inverse relationship called inContainer. Each of these entities is presented in an NSTableView (bound to an NSArrayController) that shows their name and a couple of other parameters.

My Problem:
I'd like to filter the list of containees based on whichever container is selected in its list. So, clicking on a particular container in the list will limit the items displayed in the containees list. Now, I can use setFilterPredicate on the containee NSArrayController to filter based on name or some other local parameter, but I can't get it to filter on the relationship.

I'm sure that I'm missing something really obvious.
How do I do it?

Thanks for any insight you can provide, timbos