             fatalError("could not load passes, wrong data type")
         }
         
+        assert(passes.count > 0, "passes should not be zero")
+        
         self.passes = passes
         
         NSNotificationCenter.defaultCenter().postNotificationName(PassesDidChangeNotification, object: nil)

    }
    
    internal func loadOrCreateInitialModel() {
        
        let url = NSURL.applicationDocumentsDirectory().URLByAppendingPathComponent(modelName)
        if NSFileManager.defaultManager().fileExistsAtPath(url.path!) == false {
            createInitialModelAtURL(url)
        }
        
        guard let data = NSData(contentsOfURL: url) else {
            fatalError("model data not found")
        }
        

            "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)
    }
}
