title: iOS: NSPredicate for "@count == 0" for both to-one and to-many relationships
body: <p>I'd like to write a single <code>NSPredicate</code> that would work for both to-one and to-many relationships:</p>
  
  <p>1) I have an <code>NSMutableArray</code> that will run the <code>filterUsingPredicate</code> method <br><br>
  2) The NSPredicate will filter out a selected relationship that contain any objects in the relationship (again, for both to-one and to-many relationships).</p>
  
  <p>I've tried something like <code>predicateWithFormat:@"relationshipObjects.@count == 0"</code>, but this only seems to work for to-many relationships. Is there a similar predicate format that will work for both to-one and to-many relationships?</p>