symfony2:使用递归迭代器迭代器的嵌套集

我在本教程中遇到问题：https://wildlyinaccurate.com/simple-nested-sets-in-doctrine-2

这是我的代码。正如您在评论中看到的，我无法迭代。.

<?php
class Item
{
    /**
    * @ORM'Id
    * @ORM'Column(type="integer")
    * @ORM'GeneratedValue(strategy="AUTO")
    */
    protected $id;
    /**
    * @ORM'OneToMany(targetEntity="Item", mappedBy="parent", cascade={"remove"})
    * @ORM'OrderBy({"position" = "ASC"})
    */
    private $children;
    /**
    * @Gedmo'SortableGroup
    * @ORM'ManyToOne(targetEntity="Item", inversedBy="children")
    * @ORM'JoinColumn(name="parent_id", referencedColumnName="id", nullable=true)
    */
    private $parent;
    ...

///////////////////////////////////////
namespace Project'BackendBundle'Entity;
use Doctrine'Common'Collections'Collection;
class RecursiveCategoryIterator implements 'RecursiveIterator
{
    private $_data;
public function __construct(Collection $data)
{
    $this->_data = $data;
}
public function hasChildren()
{
    return ( ! $this->_data->current()->getChildren()->isEmpty());
}
public function getChildren()
{
    return new RecursiveCategoryIterator($this->_data->current()->getChildren());
}
public function current()
{
    return $this->_data->current();
}
public function next()
{
    $this->_data->next();
}
public function key()
{
    return $this->_data->key();
}
public function valid()
{
    return $this->_data->current() instanceof Project'BackendBundle'Entity'Item;
}
public function rewind()
{
    $this->_data->first();
}
}
/////////////////////////////////
$repository = $this->getDoctrine()->getRepository('ProjectBackendBundle:Item');
$root_categories = $repository->findBy(array('parent' => null));
var_dump(count($root_categories));// this returns 2
$collection = new 'Doctrine'Common'Collections'ArrayCollection($root_categories);
$category_iterator = new RecursiveCategoryIterator($collection);
$recursive_iterator = new 'RecursiveIteratorIterator($category_iterator);
var_dump($recursive_iterator->hasChildren()); //returns true
foreach ($recursive_iterator as $index => $child_category)
{
  die("jfks"); //this is not shown
}