title: Custom NSEntityMigrationPolicy relation
body: <p>I'm trying to make a database upgrade using a custom <code>NSEntityMigrationPolicy</code>. This is how my data model looks like now and what is the intended result : </p>
  
  <pre><code>  A &lt;--------&gt;&gt; B   ( 1 A -&gt; many B ) 
  </code></pre>
  
  <p>I want a new C entity which will "break" the B type objects in some subsets : </p>
  
  <pre><code>  A &lt;--------&gt;&gt; C &lt;-------&gt;&gt; B  ( 1 A can have many C, each C can have many B )
  </code></pre>
  
  <p>I made a custom <code>NSEntityMigrationPolicy</code> for entity A. I created some C entities for each A in <code>createDestinationInstancesForSourceInstance:entityMapping:manager:error:</code> , but I'm stuck what to do with the relation between A and B. </p>
  
  <p>In docs they say to use <code>createRelationshipsForDestinationInstance</code> somehow to create the new relations. I haven't found any suggestive examples however for this case. I suppose probably the A &lt;->> C relation can be created here, but the B's may not exist yet in the destination context to create a C&lt;-->>B relation yet. How can a migration like this be done?  Please, help :)</p>
  
  <p>Thanks !</p>