public function extractJson()
    {
        foreach($this->getRows() as &$row) {

            foreach($this->getJsonFields() as $field) {
                if (isset($row[$field])) {

                    $data = json_decode($row[$field], JSON_OBJECT_AS_ARRAY);

                    foreach((array) $data as $k => $v) {
                        $row[sprintf('_%s__%s', $field, $k)] = $v;
                    }
                }
            }
        }
        return $this;
    }