code: func (aug *AuthUserGroup) Update(db XODB) error {
  	var err error
  
  	// if doesn't exist, bail
  	if !aug._exists {
  		return errors.New("update failed: does not exist")
  	}
  
  	// if deleted, bail
  	if aug._deleted {
  		return errors.New("update failed: marked for deletion")
  	}
  
  	// sql query
  	const sqlstr = `UPDATE public.auth_user_groups SET (` +
  		`user_id, group_id` +
  		`) = ( ` +
  		`$1, $2` +
  		`) WHERE id = $3`
  
  	// run query
  	XOLog(sqlstr, aug.UserID, aug.GroupID, aug.ID)
  	_, err = db.Exec(sqlstr, aug.UserID, aug.GroupID, aug.ID)
  	return err
  }
docs: // Update updates the AuthUserGroup in the database.