title: Updating multiple models during end stage of NSEntityMigrationPolicy
body: <p>I'm writing my first NSEntityMigrationPolicy right now and this one is pretty simple.  It fetches a singleton from the DB and then churns through records of <strong>three</strong> other types and links them to the singleton for a new relationship that's been added to these classes.</p>
  
  <p>Right now, I'm doing this in a single subclass of NSEntityMigrationPolicy in the <code>endEntityMapping:manager:error:</code> method, which updates objects of all three types at once.  The thing I'm unsure about is whether or not it's safe to do all of them at once, or whether I actually need to implement three separate policies ... it seems like the <code>endEntityMapping</code> callback is called after all the migrations are done, and even if I set the mapping name into the "Custom Policy" field of all three classes in the mapping model editor, it only seems to be called once.</p>
  
  <p>Just wanted to check whether or not doing everything in a single <code>endEntityMapping</code> impl like this is safe?  It certainly seems to be, but this is something I want to be sure about.</p>
  
  <p>Thanks in advance for any help.</p>