title: CoreData: Merge two contexts before saving the document
body: <p>I'm writing Coredata document-based application (i.e. CoreData+NSPersistentDocument): as I create a new document, a new coredata stack is initialized, with a model and a single managed context.</p>
  
  <p>Before the user saves the document, what I need to do is:
  - create a new context
  - create many new managed objects in it
  - merge the two contexts.</p>
  
  <p>I issue save: on the new context, but it raises the exception "This NSPersistentStoreCoordinator has no persistent stores.  It cannot perform a save operation.".</p>
  
  <p>How I can do? 
  Should I need to save the document before this operation?</p>
  
  <p>By the way, if I save the document, create a new context, import data and save the new context and finally save the document context, I get the following message: "This document’s file has been changed by another application since you opened or saved it."; any ideas?</p>
  
  <p>Best regards!</p>