public function sum($column = null)
    {
        $this->prepare();

        $sum = 0;
        if (is_null($column)) {
            $sum = array_sum($this->_map);
        } else {
            foreach ($this->_map as $key => $val) {
                $value = $this->getFromNested($val, $column);
                if (is_scalar($value)) {
                    $sum += $value;
                }

            }
        }

        return $sum;
    }