code: func (aug *AuthUserGroup) Insert(db XODB) error {
  	var err error
  
  	// if already exist, bail
  	if aug._exists {
  		return errors.New("insert failed: already exists")
  	}
  
  	// sql query
  	const sqlstr = `INSERT INTO django.auth_user_groups (` +
  		`user_id, group_id` +
  		`) VALUES (` +
  		`:1, :2` +
  		`) RETURNING id /*lastInsertId*/ INTO :pk`
  
  	// run query
  	XOLog(sqlstr, aug.UserID, aug.GroupID, nil)
  	res, err := db.Exec(sqlstr, aug.UserID, aug.GroupID, nil)
  	if err != nil {
  		return err
  	}
  
  	// retrieve id
  	id, err := res.LastInsertId()
  	if err != nil {
  		return err
  	}
  
  	// set primary key and existence
  	aug.ID = float64(id)
  	aug._exists = true
  
  	return nil
  }
docs: // Insert inserts the AuthUserGroup to the database.