code: def on_site(self, site_id=None):
          
          if settings.PAGE_USE_SITE_ID:
              if not site_id:
                  site_id = settings.SITE_ID
              return self.filter(sites=site_id)
          return self.all()
docs: Return a :class:`QuerySet` of pages that are published on the site
          defined by the ``SITE_ID`` setting.
  
          :param site_id: specify the id of the site object to filter with.