Enabling a button using data from Parse.com - Swif

Question

I am trying to enable a bar button if the user is labeled "Yes" in the SubscribedUser key. I really do not know how to do this. I have used queries before but only to call objects onto the screen, nothing like this before.

Here is what I have right now, I know it is wrong but I hope you get the idea of what I'm trying to do right now:

func enableButton() {

    var query: PFQuery = PFQuery(className: "User")

    query.whereKey("username", equalTo: PFUser.currentUser().username)


    if query.valueForKey("SubscribedUser") = "Yes"{

        self.barButton.enabled = true

    }else{

        self.barButton.enabled = false
    }



}