public function compileJson()
    {
        $report = $this->internalCompile();
        if ($report->getType() == common_report_Report::TYPE_SUCCESS) {
            // replace instances with strign identifiers
            list($item, $publicDirectory, $privateDirectory) = $report->getData();
            $report->setData([$item->getUri(), $publicDirectory->getId(), $privateDirectory->getId()]);
        }
        return $report;
    }