Exception thrown in NSOrderedSet generated accessors

Question

On my Lion app, I have this data model:

The relationship subitem

Answer 1

Better version of the correct answer in SWIFT

var tempSet = NSMutableOrderedSet()
if parent!.subItems != nil {
    tempSet = NSMutableOrderedSet(orderedSet: parent!.subItems!)
}

tempSet.add(newItem)
parent!.subItems = tempSet