Parameterised Core Data fetch predicate in NSArrayController?

FROM : Luke Evans
DATE : Thu Feb 14 23:30:43 2008

Dear Cocoa list,

I'm wondering if there's a straightforward was to have an 
NSArrayController make a Core Data fetch for its content where the 
fetch predicate can match against a parameter bound to some selection 
in another control.

In my case I have a list of objects of class A in a control with 
single selection, and would like another control to be populated with 
Core Data items of a particular entity, where some attribute is 
related to something in the selected A.  The fetch predicate could of 
course be something like:
attribute == A.foo
and the Entity name set to the right kind (having 'attribute' as an 
attribute).  However, the key 'A.foo' would need to be reachable, and 
I don't think there's any binding I can make that simply gets the 
selection of another control into the scope of this one so that I 
could address a key path on it.

Clearly, I'm trying to be 'frugal' here.  I'm aware that I can achieve 
what I want by binding the content to, say, a Window Controller key 
that has the appropriate logic in its get accessor to generate the 
list of objects I want (from having a binding to the selection in the 
'upstream' control).

I suppose my question boils down to the following:
If you want to 'restart' the master-detail chain between controls, 
because there's no way to simply refer to a child (model key) of a 
selection binding, do you essentially have to resort to some 
'external' logic to do this?

It strikes me that if I could have a 'general' binding to the upstream 
selection, then I may be able to refer to this via a key in my filter 
predicate and thus start a new 'master' set of objects.

Cheers

-- Lwe