title: Add Core Data Index to certain Attributes via migration
body: <p>For performance reasons, i want to set the <strong>Indexed Attribute</strong> to some of my entities. I created a new core data model version to perform the changes. Core Data detects the changes and migrates my model to the new version, however, <strong>NO INDEXES ARE GENERATED</strong>.</p>
  
  <p>If I recreate the database from scratch, the indexes are there. 
  I checked with SQLite Browser both on the iPhone and on the Simulator.
  The problem only occurs if a database in the prior format is already there.</p>
  
  <p>Is there a way to manually add the indexes? Write some sql for that? Or am I missing something? I did already some more critical migrations, no problems there. But those missing indexes are bugging me.</p>
  
  <p>Thanks for helping!</p>