def group_by(self, *args):
for name in args:<EOL><INDENT>assert name in self._fields or name in self._calculated_fields,'<STR_LIT>' % name<EOL><DEDENT>qs = copy(self)<EOL>qs._grouping_fields = args<EOL>return qs<EOL>
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.