orm.go
func (idx *Index) Fields() map[string]*Field {
	result := make(map[string]*Field)
	for k, v := range idx.fields {
		result[k] = v.copy()
	}
	return result
}
func (idx *Index) Fields() map[string]*Field {
	result := make(map[string]*Field)
	for k, v := range idx.fields {
		result[k] = v.copy()
	}
	return result
}
// Fields return a copy of the fields in this index