It is a well known bug, when a relationship is checked with ordered, some Dynamically-Generated accessor Methods (add*) will throw exception, some others (insert*) are just not generated.
I ran into this issue again. Yeah, I met it about 1 year ago. I’m one hundred percent sure it is a bug. And a patch is created by someone long ago https://github.com/CFKevinRef/KCOrderedAccessorFix
But, it seems Apple guys had decided not to fix it. The radar item are closed now!
Fortunately, -mutableOrderedSetValueForKey: method mentioned by document always works. I guess it is the time for Apple to update their document (https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/CoreData/Articles/cdUsingMOs.html#//apple_ref/doc/uid/TP40001803-SW3) :
Typically, however, you do not want to set an entire relationship, instead you want to add or remove a single element at a time. To do this, you should use
mutableSetValueForKey:or one of the automatically-generated relationship mutator methods (see Dynamically-Generated Accessor Methods): unless it is order!