def on_site(self, site_id=None):
        """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.
        """
        if settings.PAGE_USE_SITE_ID:
            if not site_id:
                site_id = settings.SITE_ID
            return self.filter(sites=site_id)
        return self.all()