docs: def get_sibling(self, feed_id, sibling_type):
          """
          get a previous/next sibling from a feed
          :param feed_id: id of the feed
          :param sibling_type: sibling type ('previous', 'next')
          :return: the sibling
          """
          if sibling_type == 'previous':
              query = self.session.query(self.feed_model).filter(self.feed_model.id < feed_id)
              order_clause = desc(self.feed_model.id)
          elif sibling_type == 'next':
              query = self.session.query(self.feed_model).filter(self.feed_model.id > feed_id)
              order_clause = self.feed_model.id
          else:
              raise Exception('Unhandled sibling relation type')  # pragma no cover
          if query.count() > 0:
              return query.order_by(order_clause).first()
          else:
              return None