title: Core data don't return full objects, data: <fault>
body: <p>I'm trying to fetch two entities eg: Student with <code>email</code> <code>attribute</code> and Professor, both of have same parent entity eg: Person with <code>attributes</code> <code>entityId, firstName and lastName</code> i want to generate them in two sections using <code>NSFetchedResultsController</code>.
  Here is a part from getter for <code>fetchedResultsController</code></p>
  
  <pre><code>NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
  
  [fetchRequest setFetchBatchSize:20];
  [fetchRequest setReturnsObjectsAsFaults:NO];
  
  NSEntityDescription *description = [NSEntityDescription entityForName:@"Person"
                                                 inManagedObjectContext:self.managedObjectContext];
  [fetchRequest setEntity:description];
  
  NSSortDescriptor *firstNameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"firstName" ascending:YES];
  NSSortDescriptor *lastNameDescriptor = [[NSSortDescriptor alloc] initWithKey:@"lastName" ascending:YES];
  [fetchRequest setSortDescriptors:@[firstNameDescriptor, lastNameDescriptor]];
  
  NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc]
                                                           initWithFetchRequest:fetchRequest
                                                           managedObjectContext:self.managedObjectContext
                                                             sectionNameKeyPath:@"entityId"
                                                                      cacheName:nil];
  </code></pre>
  
  <p>All <code>Students</code> have the same <code>entityId</code> and all <code>Professors</code> too</p>
  
  <p>In tableView I have two prototype cells one for <code>Student</code> and another for <code>Professor</code>.
  <a href="https://i.stack.imgur.com/MSNMQ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MSNMQ.png" alt="It looks smth like this"></a></p>
  
  <p>I get two sections as expected but students are in different sections, i have printed all objects from <code>fetchedResultsController</code> in console like this, <code>po [self.fetchedResultsController objectAtIndexPath: [NSIndexPath indexPathForItem:1 inSection:1]]</code> all professors are printed with <code>fault</code>: </p>
  
  <pre><code>&lt;Professor: 0x6080000befc0&gt; (entity: Professor; id: 0xd0000000001c0002 &lt;x-coredata://03A3ECAD-CCA7-424E-86F9-258D25372BA1/Professor/p7&gt; ; data: &lt;fault&gt;)
  </code></pre>
  
  <p>I have forced the fetch request to return full objects using <code>[request setReturnsObjectsAsFaults:NO]</code> but it had no effect.</p>
  
  <p>Why is it happening so?</p>