code: def auto_track_url(track):
      
      hub = track.root(cls=Hub)
  
      if hub is None:
          raise ValueError(
              "track is not fully connected because the root is %s" % repr(hub))
      if hub.url is None:
          raise ValueError("hub.url is not set")
      if track.source is None:
          raise ValueError("track.source is not set")
docs: Automatically sets the bigDataUrl for `track`.
  
      Requirements:
  
          * the track must be fully connected, such that its root is a Hub object
          * the root Hub object must have the Hub.url attribute set
          * the track must have the `source` attribute set