title: Core data migration failing with "Can't find model for source store" but managedObjectModel for source is present
body: <p>I have a cocoa application using core-data, which is now at the 4th version of its managed object model. </p>
  
  <p>My managed object model contains abstract entities but so far I have managed to get migration working by creating appropriate mapping models and creating my persistent store using addPersistentStoreWithType:configuration:options:error and with the NSMigratePersistentStoresAutomaticallyOption set to YES.</p>
  
  <pre><code>NSDictionary *optionsDictionary = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:NSMigratePersistentStoresAutomaticallyOption];
  NSURL *url = [NSURL fileURLWithPath: [applicationSupportFolder stringByAppendingPathComponent: @"MyApp.xml"]];
  NSError *error=nil;
  [theCoordinator addPersistentStoreWithType:NSXMLStoreType configuration:nil URL:url options:optionsDictionary error:&amp;error]
  </code></pre>
  
  <p>This works fine when I migrate from model version 3 to 4, which is a migration that involves adding attributes to several entities.   Now when I try to add a new model version (version 5), the call to addPersistentStoreWithType returns nil and the error remains empty.  The migration from 4 to 5 involves adding a single attribute. </p>
  
  <p>I am struggling to debug the problem and have checked all the following;</p>
  
  <ol>
  <li><p>The source database is in fact at version 4 and the persistentStoreCoordinator's managed object model is at version 5.</p></li>
  <li><p>The 4->5 mapping model as well as managed object models for versions 4 and 5 are present in the resources folder of my built application.</p></li>
  <li><p>I've tried various model upgrade paths.  Strangely I find that upgrading from an early version 3 -> 5 works .. but upgrading from 4 -> 5 fails.</p></li>
  <li><p>I've tried adding a custom entity migration policy for migration of the entity whose attributes are changing ... in this case I overrode the method beginEntityMapping:manager:error: .  Interestingly this method does get called when migration works (ie when I migrate from 3 to 4, or from 3 to 5 ), but it does not get called in the case that fails ( 4 to 5 ). </p></li>
  </ol>
  
  <p>I'm pretty much at a loss as to where to proceed.  Any ideas to help debug this problem would be much appreciated.  </p>