code: func (g *GormDB) SetTransactionIsolationLevel(level TXIsoLevel) error {
  	switch level {
  	case TXIsoLevelReadCommitted:
  		g.txIsoLevel = "READ COMMITTED"
  	case TXIsoLevelRepeatableRead:
  		g.txIsoLevel = "REPEATABLE READ"
  	case TXIsoLevelSerializable:
  		g.txIsoLevel = "SERIALIZABLE"
  	case TXIsoLevelDefault:
  		g.txIsoLevel = ""
  	default:
  		return fmt.Errorf("Unknown transaction isolation level: " + strconv.FormatInt(int64(level), 10))
  	}
  	return nil
  }
docs: // SetTransactionIsolationLevel sets the isolation level for
  // See also https://www.postgresql.org/docs/9.3/static/sql-set-transaction.html