title: NSFetchedResultsController sorted by a one-to-many relation
body: <p>I have three object types in CoreData; ObjectA, ObjectB and ObjectC. These objects are related with one-to-many relations like this:</p>
  
  <p><code>[ObjectA]--E[ObjectB]--E[ObjectC]</code></p>
  
  <p>What I try to accomplish, is to make a NSFetchedResultsController grouped by a property in ObjectA, and sorted by a property in ObjectC. </p>
  
  <p>How the sort should work:
  - In ObjectC there is a date property; lastUpdated. ObjectA will have a connection to several ObjectC´s, and by that several lastUpdated´s.
  - My NSFetchedResultsController will display ObjectA, sorted by these lastUpdated´s.
  - The ObjectA with the most recent updated ObjectC should appear first. 
  - I am just interested in the ObjectC of an ObjectA with the most recent lastUpdated value.</p>
  
  <p>Is this possible with NSFetchedResultsController and CoreData?</p>
  
  <p>I assume that an alternative is to make a NSFetchedProperty on ObjectA reffering to ObjectC, but that would perhaps not be a better solution. </p>