title: Sort descriptors with NSFetchedResultsController - Swift
body: <p>I have a <code>UITableView</code> fed from <code>Core Data</code> with an <code>NSFetchedResultsController</code> returning the Location entities. The default sorting (and section headers) is via the first letter of the entity's name. This works (albeit I'm still trying to combine upper and lower case into the same section properly.) The user can choose to order the table by one of three optional categories (which are attributes of the entity) and then these categories are sorted by entity name.</p>
  
  <p>When I set to sorting by category I get the following runtime error:</p>
  
  <pre><code>[_TtCSs23_ContiguousArrayStorage00007F80513B59D0 key]: unrecognized selector sent to instance 0x7f80513b5720
  </code></pre>
  
  <p>This is my <code>NSFetchedResultsController</code>:</p>
  
  <pre><code>var sectionNameKeyPathString1: String?
  var sectionNameKeyPathString2: String?
  
  var fetchedResultsController: NSFetchedResultsController {
      if _fetchedResultsController != nil {
          return _fetchedResultsController!
      }
  
      let fetchRequest = NSFetchRequest()
      // Edit the entity name as appropriate.
      let entity = NSEntityDescription.entityForName("Location", inManagedObjectContext: self.managedObjectContext!)
      fetchRequest.entity = entity
  
      // Set the batch size to a suitable number.
      fetchRequest.fetchBatchSize = 20
  
      // Edit the sort key as appropriate.
      if sectionNameKeyPathString1 != nil {
          let sortDescriptor1 = NSSortDescriptor(key: sectionNameKeyPathString1!, ascending: true)
          let sortDescriptor2 = NSSortDescriptor(key: sectionNameKeyPathString2!, ascending: true)
          let sortDescriptors = [sortDescriptor1, sortDescriptor2]
          fetchRequest.sortDescriptors = [sortDescriptors]
      } else {
          let sortDescriptor = NSSortDescriptor(key: "locationName", ascending: true)
          fetchRequest.sortDescriptors = [sortDescriptor]
      }
  
      var sectionNameKeyPath: String
      if sectionNameKeyPathString1 == nil {
          sectionNameKeyPath = "firstLetterAsCap"
      } else {
          sectionNameKeyPath = sectionNameKeyPathString1!
      }
  
      // Edit the section name key path and cache name if appropriate.
      // nil for section name key path means "no sections".
      let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: self.managedObjectContext!, sectionNameKeyPath: sectionNameKeyPath, cacheName: "Locations")
      aFetchedResultsController.delegate = self
      _fetchedResultsController = aFetchedResultsController
  
      var error: NSError? = nil
      if !_fetchedResultsController!.performFetch(&amp;error) {
          // TODO: Handle this error
          // Replace this implementation with code to handle the error appropriately.
          // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
          //println("Unresolved error \(error), \(error.userInfo)")
          abort()
      }
  
      return _fetchedResultsController!
  }
  </code></pre>
  
  <p>Using breakpoints I am confident that, for example, sectionNameKeyPathString1 = "category1" and sectionNameKeyPathString2 = "locationName" and also that sectionNameKeyPath = "category1" so the key path matches the first sort descriptor.</p>
  
  <p>I had this working in Obj-C but now I'm pulling my hair out and sure I'm suffering from bug-blindness.</p>