title: Custom Policy in my xcmappingmodel will not be executed
body: <p>I'm trying to create a mapping model for my app that has some custom code. Thus I created a xcmappingmodel and I subclassed NSEntityMigrationPolicy and implemented the</p>
  
  <pre><code>createDestinationInstancesForSourceInstance
  </code></pre>
  
  <p>method. Further I entered the classname in the Custom Policy field in my mapping model. Unfortunately my code will never be called (tried breakpoints and logs). My store is created with the following options:</p>
  
  <pre><code>NSMigratePersistentStoresAutomaticallyOption: YES
  NSInferMappingModelAutomaticallyOption: YES
  </code></pre>
  
  <p>I'm using XCode 4.0.2 (and tested with XCode 4.2 too). All other of my "lightweight" migrations are running smoothly. What do I need to do to get my custom migration running?</p>