public function exportTrackerData($tracker_id)
 {
     $artifacts_node = $this->node_helper->createElement('artifacts');
     foreach ($this->dao->searchArtifacts($tracker_id) as $row) {
         $artifact_exporter = new ArtifactXMLExporterArtifact($this->dao, $this->attachment_exporter, $this->node_helper, $this->logger);
         $artifact_node = $artifact_exporter->exportArtifact($tracker_id, $row);
         $artifacts_node->appendChild($artifact_node);
     }
     $this->node_helper->appendChild($artifacts_node);
 }