title: FetchedResultsController NSSortDescriptor for one-to-many relationship
body: <p>I have 2 entities: <code>chat</code>&lt;->> <code>chatMessages</code> (each chat has 0+ messages, each message belongs to one chat).</p>
  
  <p>I'm making <code>FetchedResultsController</code> and I'd like to sort chats by date of last message:</p>
  
  <pre><code>    NSEntityDescription *entity = [NSEntityDescription entityForName:@"ChatModel" inManagedObjectContext:[NSManagedObjectContext MR_defaultContext]];
      [fetchRequest setEntity:entity];
  
      NSSortDescriptor *sort = [[NSSortDescriptor alloc] initWithKey:@"chatMessages.messageDate" ascending:NO];
      [fetchRequest setSortDescriptors:[NSArray arrayWithObject:sort]];
  </code></pre>
  
  <p>But with this code I get: </p>
  
  <pre><code>*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'to-many key not allowed here'
  </code></pre>
  
  <p>How can I sort chats in required way? (just like whatsapp)</p>