Retrieve user-agents, sorted by most common to least common.

def user_agents(self):
        """
        Retrieve user-agents, sorted by most common to least common.
        """
        return (self.get_query()
                .select(
                    PageView.headers['User-Agent'],
                    fn.Count(PageView.id))
                .group_by(PageView.headers['User-Agent'])
                .order_by(fn.Count(PageView.id).desc())
                .tuples())