title: CoreData - "join" rows from the same entity
body: <p>I'm quite new to Core Data and want to apologise in advance if i wil use sql terminology only to try to explain my question better. I did search the net but didn't find answer to it in some clean manner. Is it possible with Core Data to 'join' two rows - at search time - from the same entity that have some unique 'external' id (not the one provided by core data at insertion time). For exaple i have one table that has fields:</p>
  
  <pre><code>unique_id | description      | iso_code 
  ----------------------------------------
  13        | some desc        | code A 
  13        | some other stuff | code B 
  13        | some foo         | code C
  
  ----------------------------------------
  16        | some fun desc    | code A 
  16        | some other stuff | code B 
  16        | some foo bar     | code C
  </code></pre>
  
  <p>So every row has the same id of 13 or other, I would want to "join" such sets of same id at fetch time and for example sort such joined result sets by description for example - where  iso_code is code C. then display the results in table view.</p>
  
  <p>Is possible to do this without additional multiple array and dictionary manipulations in memory at runtime - eg getting 3 x more results then combining them into reduced filtered collection that will be displayed. Or without using one entity more and create relations with it.</p>