title: Swift Core Data, dynamic cast fail when setting relationship
body: <p>Ive been stuck for a couple days, and need some help.
  My data model is setup with two entities(one to many relationship). Round &lt;---->> Hole. In one view controller, I setup the Round and assign its properties(lets call this object roundObject). I do a fetch and grab this NSManagedObject roundObject and pass it on to a different View Controller. In this new VC, I want to now assign the properties of Hole(which has a relationship with Round). However, when I try to assign the relationship attribute of Hole( which is named holeToRound) it will not accept the NSManagedObject roundObject that I have passed in. It tells me that "NSManagedObject is not convertible to Round". So I tried casting MyRound as Round, and it then crashes the app telling me "Swift dynamic cast failed".</p>
  
  <p>Any help would be great. Am I just missing a basic step in setting relationship properties? Essentially, I want to be able to set one entity properties in one VC, and the other entity via one-to-many relationship attributes in another VC. For instance, in this particular app, I have a single Golf Round where i define the Course name, and then I have MANY Hole objects (1-18) where I want to define and set the par and yardage for each hole.</p>
  
  <p>Here is the two Model Classes</p>
  
  <pre><code>class Hole: NSManagedObject {
  
  @NSManaged var par: String
  @NSManaged var yards: String
  @NSManaged var holeToRound: Round
  @NSManaged var holeNum: String
  
  }
  
  class Round: NSManagedObject {
  
  @NSManaged var course: String
  @NSManaged var playerOne: String
  @NSManaged var playerTwo: String
  @NSManaged var roundToHole: NSSet
  
  }
  </code></pre>
  
  <p>And this is the prepareFroSegue where I am trying to assign the relationship attribute </p>
  
  <pre><code>override func prepareForSegue(segue: UIStoryboardSegue!, sender: AnyObject!) {
      let appDel:AppDelegate = UIApplication.sharedApplication().delegate as AppDelegate
      let context:NSManagedObjectContext = appDel.managedObjectContext
  
      if segue.identifier == "hole1Segue" {
  
  
  
          let entity = NSEntityDescription.entityForName("Hole", inManagedObjectContext: context)
  
          let newHole = Hole(entity: entity, insertIntoManagedObjectContext: context)
  
  
  
          newHole.holeToRound = roundObject as Round
  
          context.save(nil)
  }
  </code></pre>