Swift Core Data Batch Обновление создания дубликатов записей вместо перезаписи

Кажется, что мой NSPredicate не работает при обновлении записей Core Data. При выполнении запроса выборки, тот же NSPredicate работает без проблем.

Когда я делаю Batch Update, он просто создает новые повторяющиеся записи вместо того, чтобы перезаписывать существующие, как предполагалось. Почему, почему?

Вот мой код, который выполняет обновление:

let appDelegate = UIApplication.sharedApplication().delegate as AppDelegate

lazy var managedObjectContext : NSManagedObjectContext? = {
    if let managedObjectContext = self.appDelegate.managedObjectContext {
        return managedObjectContext
    }
    else {
        return nil
    }
    }()

func doesMessageExist(id: String) -> Bool {
    let fetchRequest = NSFetchRequest(entityName: "ChatMessage")
    let predicate = NSPredicate(format: "id == %@", id)
    fetchRequest.predicate = predicate
    fetchRequest.fetchLimit = 1

    let count = managedObjectContext!.countForFetchRequest(fetchRequest, error: nil)
    return (count > 0) ? true : false
}

func updateMessage(chatMessage: ChatMessage) {
    var batchRequest = NSBatchUpdateRequest(entityName: "ChatMessage")

    if doesMessageExist(chatMessage.id) {
        batchRequest.predicate = NSPredicate(format: "id == %@", chatMessage.id)
    }

    batchRequest.propertiesToUpdate = [
        "id" : chatMessage.id,
        "senderUserId" : chatMessage.senderUserId,
        "senderUsername" : chatMessage.senderUsername,
        "receiverUserId" : chatMessage.receiverUserId,
        "receiverUsername" : chatMessage.receiverUsername,
        "messageType" : chatMessage.messageType,
        "message" : chatMessage.message,
        "timestamp" : chatMessage.timestamp
    ]

    batchRequest.resultType = .UpdatedObjectsCountResultType
    var error : NSError?
    var results = self.managedObjectContext!.executeRequest(batchRequest, error: &error) as NSBatchUpdateResult
    if error == nil {
        println("Update Message: \(chatMessage.id) \(results.result)")
        appDelegate.saveContext()
    }
    else {
        println("Update Message Error: \(error?.localizedDescription)")
    }
}

Вот мой класс ChatMessage:

class ChatMessage: NSManagedObject {

    @NSManaged var id: String
    @NSManaged var message: String
    @NSManaged var messageType: String
    @NSManaged var receiverUserId: String
    @NSManaged var receiverUsername: String
    @NSManaged var senderUserId: String
    @NSManaged var senderUsername: String
    @NSManaged var timestamp: NSDate

}

Вот стек Core Data в моем AppDelegate:

lazy var applicationDocumentsDirectory: NSURL = {
    // The directory the application uses to store the Core Data store file. This code uses a directory named "com.walintukai.LFDate" in the application documents Application Support directory.
    let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
    return urls[urls.count-1] as NSURL
}()

lazy var managedObjectModel: NSManagedObjectModel = {
    // The managed object model for the application. This property is not optional. It is a fatal error for the application not to be able to find and load its model.
    let modelURL = NSBundle.mainBundle().URLForResource("LFDate", withExtension: "momd")!
    return NSManagedObjectModel(contentsOfURL: modelURL)!
}()

lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator? = {
    // The persistent store coordinator for the application. This implementation creates and return a coordinator, having added the store for the application to it. This property is optional since there are legitimate error conditions that could cause the creation of the store to fail.
    // Create the coordinator and store
    var coordinator: NSPersistentStoreCoordinator? = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
    let url = self.applicationDocumentsDirectory.URLByAppendingPathComponent("LFDate.sqlite")
    var error: NSError? = nil
    var failureReason = "There was an error creating or loading the application saved data."
    if coordinator!.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: url, options: nil, error: &error) == nil {
        coordinator = nil
        // Report any error we got.
        let dict = NSMutableDictionary()
        dict[NSLocalizedDescriptionKey] = "Failed to initialize the application saved data"
        dict[NSLocalizedFailureReasonErrorKey] = failureReason
        dict[NSUnderlyingErrorKey] = error
        error = NSError(domain: "YOUR_ERROR_DOMAIN", code: 9999, userInfo: dict)
        // Replace this 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.
        NSLog("Unresolved error \(error), \(error!.userInfo)")
        abort()
    }

    return coordinator
}()

lazy var managedObjectContext: NSManagedObjectContext? = {
    // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.) This property is optional since there are legitimate error conditions that could cause the creation of the context to fail.
    let coordinator = self.persistentStoreCoordinator
    if coordinator == nil {
        return nil
    }
    var managedObjectContext = NSManagedObjectContext()
    managedObjectContext.persistentStoreCoordinator = coordinator
    managedObjectContext.mergePolicy = NSOverwriteMergePolicy
    return managedObjectContext
}()

// MARK: - Core Data Saving support

func saveContext () {
    dispatch_async(dispatch_get_main_queue(),{
        if let moc = self.managedObjectContext {
            var error: NSError? = nil
            if moc.hasChanges && !moc.save(&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.
                NSLog("Database Save Error: \(error), \(error!.userInfo)")
                abort()
            }
        }
    });
}