class My_Form_validation extends CI_Form_validation {

    function __construct($rules = array())
    {
        parent::__construct();
        $CI =& get_instance();
        $this->_config_rules = $rules;
    }

    // --------------------------------------------------------------------

    /**
     * Unique
     *
     * @access  public
     * @param   string
     * @param   field
     * @return  bool
     */
    function unique($str, $field)
    {
        $CI =& get_instance();
        list($table, $column) = explode('.', $field, 2);

        $CI->form_validation->set_message('unique', 'The %s that you requested is unavailable.');

        $query = $CI->db->query("SELECT COUNT(*) AS dupe FROM $table WHERE $column = '$str'");
        $row = $query->row();
        return ($row->dupe > 0) ? FALSE : TRUE;
    }
}
?>