code: func (q *QueryDsl) Fields(fields, search, exists, missing string) *QueryDsl {
  	fieldList := strings.Split(fields, ",")
  	qs := NewQueryString("", "")
  	q.QueryEmbed.Qs = &qs
  	q.QueryEmbed.Qs.Query = search
  	if len(fieldList) == 1 {
  		q.QueryEmbed.Qs.DefaultField = fields
  	} else {
  		q.QueryEmbed.Qs.Fields = fieldList
  	}
  	q.QueryEmbed.Qs.Exists = exists
  	q.QueryEmbed.Qs.Missing = missing
  	return q
  }
docs: // Fields in query_string search
  //     Fields("fieldname","search_for","","")
  //
  //     Fields("fieldname,field2,field3","search_for","","")
  //
  //     Fields("fieldname,field2,field3","search_for","field_exists","")