Controller.php

<?php
// namespace and use
class DefaultController extends Controller
{
  
  public function saveAction($listId, $id)
  {
    $list = $this->getDataManager()->getRepository('BiozshockListsBundle:MyList')->find($listId);
    
    //FIXME: replace with filter.
    // workaround because $this->getDataManager()->getRepository('BiozshockListsBundle:MyList')
    // ->findOneBy(array('id' => $listId, 'item.id' => $id)) not working.. :(
    $item = null;
    /** @var Item $oneItem */
    foreach ($list->getItems() as $oneItem) {
        if ($oneItem->getId() == $id) {
            $item = $oneItem;
            break;
        }
    }
    
    if (null === $item) {
        $item = new Item();
    }
    
    $new = null === $item->getId() ? true : $list->getItems()->indexOf($item);
    
    // standard symfony form
    $form = $this->createForm(new ListType(), $item);
    $form->bind($this->getRequest());

    if ($form->isValid()) {

        $storage = $this->getDataManager();

        if ($new === true) {
            $list->addItem($item);
        } else {
            // try 1 not working
            $list[$new] = $item;
            // try 2 not working either
            $list->getItems()->remove($new);
            $list->addItem($item);
        }

        $storage->persist($item);
        $storage->persist($list);

        $storage->flush();
        
        // redirect
    }
    
    // if errors...
  }
  
}

Item.php

<?php

namespace Biozshock\ListsBundle\Document;

use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB,
    Doctrine\Common\Collections\Collection,
    Doctrine\Common\Collections\ArrayCollection
;

/**
 * Item entity
 *
 * @MongoDB\EmbeddedDocument
 *
 */
class Item
{
    /**
     * @MongoDB\Id
     */
    protected $id;
    
    /**
     * @var string
     * @MongoDB\Field(type="string")
     */
    protected $name;

    /**
     * @var string
     * @MongoDB\Field(type="string")
     */
    protected $comment;
    
    // getters and setters generated by symfony console.
    
}

List.php

<?php

namespace Biozshock\ListsBundle\Document;

use Doctrine\ODM\MongoDB\Mapping\Annotations as MongoDB,
    Doctrine\Common\Collections\Collection,
    Doctrine\Common\Collections\ArrayCollection
;


/**
 * Items entity
 *
 * @MongoDB\Document(repositoryClass="Biozshock\ListsBundle\Repository\ListRepository")
 *
 */
class MyList
{

    /**
     * @MongoDB\Id
     */
    protected $id;

    /**
     * @var \Doctrine\Common\Collections\Collection
     * @MongoDB\EmbedMany(targetDocument="Biozshock\ListsBundle\Document\Item")
     */
    protected $items;
    
    public function __construct()
    {
        $this->items = new \Doctrine\Common\Collections\ArrayCollection();
    }
    
    /**
     * Add item
     *
     * @param \Biozshock\ListsBundle\Document\Item $item
     */
    public function addItems(\Biozshock\ListsBundle\Document\Item $item)
    {
        $this->items[] = $item;
    }

    /**
     * Get items
     *
     * @return \Doctrine\Common\Collections\Collection $items
     */
    public function getItems()
    {
        return $this->items;
    }
}