def top_referrers(self, domain_only=True):
        """
        What domains send us the most traffic?
        """
        referrer = self._referrer_clause(domain_only)
        return (self.get_query()
                .select(referrer, fn.Count(PageView.id))
                .group_by(referrer)
                .order_by(fn.Count(PageView.id).desc())
                .tuples())