docs: def slice(self, order_by='pk', n=None):
          """ return n objects according to specified ordering """
          if n is not None and n < 0:
              raise ValueError('slice parameter cannot be negative')
          
          queryset = self.order_by(order_by)
          
          if n is None:
              return queryset[0]
          else:
              return queryset[0:n]