Collection View Item binding issue

Question

I have two entities in a Data Model, ENTITY_A, and ENTITY_B, that are related (ENTITY_A with a one-to-many relationship to ENTITY_B named DetailItems).

I have set up a NSCollectionView with its appropriate bindings to ENTITY_A, and have placed on a Collection View Item a label.

If I bind the label to [Collection View Item] and with a Model Key Path of [representedObject.FIELD_NAME], it works great.

If I bind it to a Model Key Path [representedObject.DetailItems.@count], again it works great. If I bind it to a Model Key Path [[email protected]_NAME], I get the following error on the console: addObserver:forKeyPath:options:context:] is not supported. Key path: @sum.FIELD_NAME.

Can anyone please help?

Thank you, Harry