private function getParentArtifacts(Planning_Milestone $milestone, PFUser $user, array $backlog_item_ids)
 {
     $parents = $this->artifact_factory->getParents($backlog_item_ids);
     $parent_tracker = $this->getParentTracker($parents);
     if ($parent_tracker) {
         $this->setParentItemName($milestone, $parent_tracker->getName());
         if ($this->userCanReadBacklogTitleField($user, $parent_tracker)) {
             $this->artifact_factory->setTitles($parents);
         } else {
             foreach ($parents as $artifact) {
                 $artifact->setTitle("");
             }
         }
     }
     return $parents;
 }