docs: def formfield_for_foreignkey(self, db_field, request=None, **kwargs):
          """
          Limit canned reply options to those with same site as comment.
          """
          field = super(CommentReplyAdmin, self).\
              formfield_for_foreignkey(db_field, request, **kwargs)
          comment_id = request.GET.get(self.fk_name, None)
  
          if db_field.name == 'canned_reply' and comment_id:
              comment_id = comment_id.split(',')
              comment_sites = Comment.objects.filter(id__in=comment_id)\
                                             .values('site')\
                                             .distinct()
              field.queryset = field.queryset.filter(Q(site__in=comment_sites) |
                                                     Q(site__isnull=True))
          return field