title: Unique Identifier for NSManagedObject
body: <p>I have a need to obtain a unique identifier for a type of <code>NSManagedObject</code> I've created. It needs to be available as soon as the object has been created, never change, and be completely unique.</p>
  
  <p>This rules out the <code>NSManagedObjectID</code>, as this can change when the context is saved. I believe the <code>-hash</code> method could be non-unique if my objects have the same properties.</p>
  
  <p>I'd really like to avoid creating an otherwise useless uniqueIdentifier UUID field on the entity as this seems wasteful and messy. Is there an accepted best practice here?</p>