Swift: Implementing Find-or-Create Efficiently - json

Question

So I set up some data from an API to create a managed object, How do I determine whether the managed object already exists (in my example, every object has a unique id), and create it if it does not.
// set up core data
var appDel:AppDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
var context:NSManagedObjectContext = appDel.managedObjectContext!
// Get data from API
let urlPath = "http://www.example.com"
let url = NSURL(string: urlPath)
let session = NSURLSession.sharedSession()
let getMovies = session.dataTaskWithURL(url!, completionHandler: {
data, response, error -> Void in
if error != nil {
println(error)
} else {
// Parse the JSON data from API
var moviesResult: NSDictionary! = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: nil) as! NSDictionary
if let subjects = moviesResult["subjects"] as? [[NSObject:AnyObject]] {
var movies = [[String:NSString]()]
var movie:AnyObject
var newMovieItem:NSManagedObject
// Loop through the arrays returned by API to get each movie
for (var i = 0 ; i < subjects.count ; i++) {
// Initialize the movies array
movies.append([String:NSString]())
movie = subjects[i] as NSDictionary
movies[i]["title"] = movie["title"] as? NSString
movies[i]["id"] = movie["id"] as? NSString
newMovieItem = NSEntityDescription.insertNewObjectForEntityForName("Movies", inManagedObjectContext: context) as! NSManagedObject
newMovieItem.setValue(movies[i]["title"], forKey: "title")
newMovieItem.setValue(movies[i]["id"], forKey: "id")
context.save(nil)
}
var request = NSFetchRequest(entityName: "Movies")
request.returnsObjectsAsFaults = false
var results = context.executeFetchRequest(request, error: nil)
}
}
})
getMovies.resume()