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If you have instantiated the NSArrayController in the xib/nib file, that is where you will see that (when you select the NSArrayController Object). If not, you will have to use a method to set it programatically.

Also, when you select the NSArrayController object in the xib, I believe that is where you select the class of the object that add: creates.
 
You were right. I deselected "Preserve Selection" and it works just fine.

The class for the NSArrayController is set for my Part class with the necessary keys to initialize the new object, so I'm still stymied about that, but I needed to create my own method to add an object, so it's ok.
 
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