Exception while registering subclass with Parse

I have a class called Attendee which inherits from PFObject. In my applicationDidFinishLaunching() method, I register the subclass like so:

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    Attendee.initialize()
    Parse.enableLocalDatastore()
    Parse.setApplicationId(ObjectManager.appID, clientKey: ObjectManager.clientKey)

    getData()

    return true
}

func getData() {
    //create the query
    var attendeeQuery = Attendee.query()
    attendeeQuery?.fromLocalDatastore()
        .fromPinWithName(CacheKeys.AttendeesKey)
        .whereKey("conference", equalTo: ObjectManager.currentConf)

    //register a background task
    var bgTask = UIBackgroundTaskInvalid
    bgTask = UIApplication.sharedApplication().beginBackgroundTaskWithExpirationHandler({ () -> Void in
        UIApplication.sharedApplication().endBackgroundTask(bgTask)
        bgTask = UIBackgroundTaskInvalid

        println("Attendees Loader Expiration Handler called")
    })

    //run the query
    attendeeQuery?.findObjectsInBackgroundWithBlock({ (cachedAttendees: [AnyObject]?, error:NSError?) -> Void in
        if error != nil {
            println(error)
            return
        }

        ...

            UIApplication.sharedApplication().endBackgroundTask(bgTask)
        })
    })
}

However, when I run the code, I get an exception at the line var attendeeQuery = Attendee.query(). The exception says that I need to register Attendee subclass before I use it. I don't understand why it is saying that, as I register it right beforehand. Below is the Attendee class definition.

class Attendee: PFObject, PFSubclassing {

    var name:String
        {
            return self["name"] as! String
    }

    ...

     override class func initialize() {
        var onceToken : dispatch_once_t = 0;
        dispatch_once(&onceToken) {
            self.registerSubclass()
        }
    }

    class func parseClassName() -> String {
        return Classes.Attendee
    }
}

The exception:

*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'The class MedConf.Attendee must be registered with registerSubclass before using Parse.'
*** First throw call stack:
(0x1835002d8 0x194d240e4 0x183500218 0x1001e21d0 0x1001e1fac 0x100135d88 0x100138af0 0x1000e43e0 0x1000e4580 0x187fb3190 0x1881ca854 0x1881cd208 0x1881cb778 0x18bd093c8 0x1834b827c 0x1834b7384 0x1834b59a8 0x1833e12d4 0x187fac43c 0x187fa6fac 0x1000e7b48 0x1953a2a08)

I had the same problem when I upgraded to Parse's iOS SDK 1.7.5 from 1.6.3. The problem was in my initialize function, which looked the same as yours.

Using a dispatch_once_t as a local var in a function doesn't serve the purpose of ensuring that the subclass is registered exactly once; I noticed by putting a breakpoint on the registerSubclass() call in the initialize function that it was being called many times. The constant re-registering of the subclass was apparently causing problems for the new version of the Parse SDK.

Changing the initialize function to look like this fixed the problem:

private static var onceToken : dispatch_once_t = 0

override class func initialize() {
    dispatch_once(&onceToken) {
        self.registerSubclass()
    }
}