NSFetchrequest创建的例外情况

Question

我正在将字典中的JSON解析为核心数据模型。当我创建一个NSFetchRequest来查找模型中的现有条目时，我得到一个例外。

AppDelegate类：

for dictionary : AnyObject in airports as AnyObject[] {
    let name : NSString? = dictionary.valueForKey("name") as? NSString
    if name {
        var airport = Airport.fromDictionary(dictionary as NSDictionary, context: managedObjectContext)
    }
}

var managedObjectContext: NSManagedObjectContext {
    if !_managedObjectContext {
        let coordinator = self.persistentStoreCoordinator
        if coordinator != nil {
            _managedObjectContext = NSManagedObjectContext()
            _managedObjectContext!.persistentStoreCoordinator = coordinator
        }
    }
    return _managedObjectContext!
}
var _managedObjectContext: NSManagedObjectContext? = nil

型号：

class func fromDictionary(dictionary: NSDictionary, context: NSManagedObjectContext) -> Airport! {
    let id = dictionary["code"] as NSString
    var airport : Airport = airportWithCode(id, context: context)
    return airport
}

class func airportWithCode(code: String, context: NSManagedObjectContext) -> Airport! {
    var fetchRequest = NSFetchRequest()
    let entityDescription = NSEntityDescription.entityForName(Airport.entityName(), inManagedObjectContext: context)
    ^.... Exception point
    fetchRequest.entity = entityDescription
    fetchRequest.predicate = NSPredicate(format: "code ==[c] %@", code)
    var error: NSError?
    let results: NSArray = context.executeFetchRequest(fetchRequest, error: &error)
    var result = results.firstObject as Airport
    if result != nil {
        return result as Airport
    } else {
        var airport = createWithCode(code, context: context) as Airport
        return airport
    }
}