private function sortPlanningsAccordinglyToHierarchy(array &$plannings)
 {
     if (!$plannings) {
         return;
     }
     $tracker_ids = array_map(array($this, 'getPlanningTrackerId'), $plannings);
     $hierarchy = $this->tracker_factory->getHierarchy($tracker_ids);
     $this->tmp_tracker_ids_to_sort_plannings = $hierarchy->sortTrackerIds($tracker_ids);
     usort($plannings, array($this, 'cmpPlanningTrackerIds'));
 }