/**
  * Get root nodes
  * @return array
  */
 public function getRootNodes()
 {
     $filter = new DoctrineSearchCondition();
     $filter->levelEqualsTo(0);
     $order = new DoctrineSelectOrder();
     $order->byLeftAscending();
     $rootNodes = $this->nestedSetRepository->search($filter, $order);
     return $rootNodes;
 }