             order += 1
         }
         
-        let data = NSKeyedArchiver.archivedDataWithRootObject(passes)
-        return data.writeToURL(url, atomically: true)
+        if didWritePasses(passes, toURL: modelURL) == false {
+            fatalError("cannot saved model to url: \(modelURL)")
+        }
     }
     
     private lazy var dateFormatter: NSDateFormatter = {


typealias PassUpdatesFuture = Future<[Pass]>

private let modelName = "passengr.plist"

class PassDataSource: NSObject {
    
    var orderedPasses: [Pass] {

            "White": "http://www.wsdot.com/traffic/passes/white/"
        ]
    }()
    
    private var modelURL: NSURL {
        let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
        return urls[urls.count - 1].URLByAppendingPathComponent("passengr.plist")
    }
    
    private func didWritePasses(passes: [Pass], toURL url: NSURL) -> Bool {
        let data = NSKeyedArchiver.archivedDataWithRootObject(passes)
        return data.writeToURL(url, atomically: true)
    }
}
