code: def group_by(self, *args):
          
          for name in args:
              assert name in self._fields or name in self._calculated_fields, \
                     'Cannot group by `%s` since it is not included in the query' % name
          qs = copy(self)
          qs._grouping_fields = args
          return qs
docs: This method lets you specify the grouping fields explicitly. The `args` must
          be names of grouping fields or calculated fields that this queryset was
          created with.