orm.go
func (s *Schema) Index(name string, options ...IndexOption) *Index {
	if index, ok := s.indexes[name]; ok {
		return index
	}
	indexOptions := &IndexOptions{}
	indexOptions.addOptions(options...)
	return s.indexWithOptions(name, 0, indexOptions)
}
func (s *Schema) Index(name string, options ...IndexOption) *Index {
	if index, ok := s.indexes[name]; ok {
		return index
	}
	indexOptions := &IndexOptions{}
	indexOptions.addOptions(options...)
	return s.indexWithOptions(name, 0, indexOptions)
}
// Index returns an index with a name.