Returns an instance of the model from a urlsafe string.

        :param urlsafe: urlsafe key
        :return: Instance of cls

def from_urlsafe(cls, urlsafe):
        """
        Returns an instance of the model from a urlsafe string.

        :param urlsafe: urlsafe key
        :return: Instance of cls
        """
        try:
            key = ndb.Key(urlsafe=urlsafe)
        except:
            return None
        obj = key.get()
        if obj and isinstance(obj, cls):
            return obj