docs: 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())