public function getChildrenPlanning(Planning $planning)
 {
     $children = $this->tracker_factory->getHierarchyFactory()->getChildren($planning->getPlanningTrackerId());
     if (count($children) == 0) {
         return null;
     } else {
         $planning_tracker = array_shift($children);
         return $this->getPlanningByPlanningTracker($planning_tracker);
     }
 }