orm.go
func (idx *Index) GroupBy(rowsQueries ...*PQLRowsQuery) *PQLBaseQuery {
	if len(rowsQueries) < 1 {
		return NewPQLBaseQuery("", idx, errors.New("there should be at least one rows query"))
	}
	text := fmt.Sprintf("GroupBy(%s)", strings.Join(serializeGroupBy(rowsQueries...), ","))
	return NewPQLBaseQuery(text, idx, nil)
}
func (idx *Index) GroupBy(rowsQueries ...*PQLRowsQuery) *PQLBaseQuery {
	if len(rowsQueries) < 1 {
		return NewPQLBaseQuery("", idx, errors.New("there should be at least one rows query"))
	}
	text := fmt.Sprintf("GroupBy(%s)", strings.Join(serializeGroupBy(rowsQueries...), ","))
	return NewPQLBaseQuery(text, idx, nil)
}
// GroupBy creates a GroupBy query with the given Rows queries