如何在Core Data中保留有序列表

I'm writing an API method that returns a list of objects from a web service. This service also caches results and attempts to return the cached results (if any) before hitting the web service again. So I'm storing this list in a Core Data entity. But Core Data only allows to-may relationships to be stored in an NSSet, which does not preserve order. But I want the cached results to retain the original order (which was originally from the web service). I don't necessarily know how that order is established (so I can't sort).I'm writing an API method that returns a list o