The problem is that NSManagedObjectID you pass is temporary. You can check it by calling NSManagedObjectID‘s isTemporaryID method. From docs:
Returns a Boolean value that indicates whether the
receiver is temporary.Most object IDs return NO. New objects inserted into a
managed object context are assigned a temporary ID which is replaced
with a permanent one once the object gets saved to a persistent store.
You should first save your changes to persistent store, only then get a permanent ID to pass to other context.