title: NSPredicate, get results with a subset of one-to-many relationship
body: <p>I'mm working around with Core Data and <code>NSFetchedResultsController</code>. </p>
  
  <p>My Data Model looks like this: </p>
  
  <p><code>Product</code> with one-to-many relationship called <code>dataLines</code>. 
  The <code>dataLine</code> entity has a property name <code>theWeek</code>. </p>
  
  <p>I want to fetch all <code>Product</code> where <code>dataLines.theWeek == someValue</code>. This is easily done with a subquery. But this returns all dataLines. Is it possible to create a <code>NSPredicate</code> that returns the <code>Product</code> and a subset if dataLines only with the <code>dataLines == someValue</code>?</p>