使用Core Data进行更新

Question

我有以下代码：

class RatingsData {

    var ratings = [NSManagedObject]()

    func fetchFromDatabase() -> Void {
        let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        let managedContext = appDelegate.managedObjectContext!
        let fetchRequest = NSFetchRequest(entityName: "Rating")
        var error: NSError?

        let sortDescriptor = NSSortDescriptor(key: "date", ascending: false)
        fetchRequest.sortDescriptors = [sortDescriptor]

        let fetchedResults = managedContext.executeFetchRequest(fetchRequest, error: &error) as! [NSManagedObject]?

        if let results = fetchedResults {
            ratings = results
        } else {
            println("error fetching from database")
        }

    }

    func addTitle(newTitle:String) -> Void {

        let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        let managedContext = appDelegate.managedObjectContext!
        let entityDescription = NSEntityDescription.entityForName("Rating", inManagedObjectContext: managedContext)

        let title = Rating(entity: entityDescription!, insertIntoManagedObjectContext: managedContext)

        //new way
        title.title = newTitle
        println("New rating title \(newTitle) saved")

        var error: NSError?
        if !managedContext.save(&error) {
            println("could not save")
        }

    }

    func addStar(title: String, star: NSData) -> Void {

        let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
        let managedContext = appDelegate.managedObjectContext!
        let entityDescription = NSEntityDescription.entityForName("Rating", inManagedObjectContext: managedContext)

        let request = NSFetchRequest()
        request.entity = entityDescription

        let predicate = NSPredicate(format: "title  = %@", title)
        request.predicate = predicate

        var error: NSError?
        var results = managedContext.executeFetchRequest(request, error: &error) as! [NSManagedObject]?

        results?[0].setValue(star, forKey: "star")
        println("new rating star saved")

        if !managedContext.save(&error) {
            println("could not save")
        }

    }
}    

评级是实体，标题和星级属性。 它工作正常，但当我点击相同的标题来更改评级时，应用程序崩溃。当点击相同的标题时，我希望能够更新星形图像并在表格视图中显示它。

我无法弄明白该怎么做。