protected function prepareHeaders($output)
    {
        $keys = $this->exportManager->getExport($this->exportAlias)->getQueryKeys($this->exportData);
        // we want to keep the order of the first row. So we will iterate on the first row of the results
        $first_row = count($this->result) > 0 ? $this->result[0] : array();
        $header_line = array();
        
        if ($this->formatterData['numerotation'] === true) {
                $header_line[] = $this->translator->trans('Number');
        }
        
        foreach ($first_row as $key => $value) {
            $header_line[] = $this->getLabel($key, '_header');
        }
        
        if (count($header_line) > 0) {
            fputcsv($output, $header_line);
        }
    }