symfony2: вложенные множества с использованием RecursiveIteratorIterator

У меня возникают проблемы после этого урока: https://wildlyinaccurate.com/simple-nested-sets-in-doctrine-2

Это мой код. Как вы можете видеть в комментариях, я не могу повторять.

<?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
}