private function add()
    {
        $columns = [];
        $values = [];
        //
        if ($this->_tracked) {
            $this->created_at = Time::current();
        }
        //
        foreach ($this->_columns as $value) {
            if ($value != $this->_keyName && isset($this->$value) && !empty($this->$value)) {
                $columns[] = $value;
                $values[] = $this->$value;
            }
        }
        //
        return $this->insert($columns, $values);
    }