title: Sort by entity name in NSFetchRequest
body: <p>I have a fetched results controller that should display all items of a certain entity that has a number of subentities. </p>
  
  <p>The sections in the fetched results controller should be based on the entity name, i.e which subentity an item belongs to. Setting the <code>sectionNameKeyPath</code> to <code>@"entity.name"</code> works.</p>
  
  <p>It seems, however, to be impossible to get the right sort descriptor for the fetch request. Things like <code>[NSSortDescriptor sortDescriptorWithKey:@"entity.name" ascending:YES]</code> result in errors like <code>keypath entity.name not found in entity Something</code>.</p>