在带有批注的表单中显示相关项

我有一个关于用形成多对多关系的实体填充我的表单的问题。

首先我的代码：

产品实体：

<?php
namespace My'Entity;
use Doctrine'ORM'Mapping as ORM;
use Doctrine'Common'Collections'ArrayCollection;
use Zend'Form'Annotation;
use My'Entity'Brand;
/**
 * @ORM'Entity(repositoryClass="My'EntityRepository'Product")
 * @ORM'Table(name="product")
 * @Annotation'Hydrator("Zend'Stdlib'Hydrator'ArraySerializable")
 * @Annotation'Name("Product")
 */
class Product{
    /**
     * @ORM'id
     * @ORM'GeneratedValue(strategy="AUTO")
     * @ORM'Column(type="integer")
     * @Annotation'Attributes({"type":"hidden"})
     */
    protected $id;
    /**
     * @ORM'ManyToMany(targetEntity="productGroup")
     * @Annotation'Type("Zend'Form'Element'Select")
     * @Annotation'Options({"label":"Productgroup: "})
     */
    protected $productGroups;

    /**
     * @ORM'Column(nullable=false)
     * @Annotation'Attributes({"type":"text"})
     * @Annotation'Options({"label":"Product name:"})
     */
    protected $productName;
    /**
     * @Annotation'Attributes({"type":"textarea"})
     * @Annotation'Options({"label":"Product description: "})
     * @ORM'Column
     */
    protected $description;
    public function __construct() {
        $this->memos = new ArrayCollection();
        $this->productGroups = new ArrayCollection();
    }
    /**
     * Sets the product tags
     * @param ArrayCollection $tags
     */
    public function setTags('Doctrine'Common'Collections'ArrayCollection $tags) {
        $this->productGroups = $tags;
    }

    /**
     * This function unsets a product group
     */
    public function unsetProductsGroups() {
        unset($this->productGroups);
    }
}

然后我有我的行动

 public function newAction()
    {
        $em = $this->getEntityManager();
        $request = $this->getRequest();
        $product = new Product();
        $builder = new AnnotationBuilder($em);
        $form = $builder->createForm($product);

        if ($request->isPost() && $this->request->getPost()) {
            $repo = $this->getEntityManager()->getRepository('My'Entity'Product');
            $repo->addProduct($this->getRequest()->getPost());
            $this->flashMessenger()->addMessage('The product was added.');
            return $this->redirect()->toRoute('zfcadmin');
        } else {
            $config = $this->getModuleConfig();
            if (isset($config['my_form_extra'])) {
                foreach ($config['my_form_extra'] as $field) {
                    $form->add($field);
                }
            }
            $form->setHydrator(new DoctrineHydrator($em, 'My'Entity'Product'));
            $form->bind($product);
            return new ViewModel(array('form' => $form));
        }
    }

和我的观点

<div class="well">
    <?php
        $form = $this->form;
        $form->setAttribute('method','post');
        echo $this->form()->openTag($form);
        echo $this->formSelect($form->get('productGroups'));
        echo $this->form()->closeTag($form);
    ?>
</div>

此输出为空选择框。即使在我的编辑表单中，设置相同但下拉列表为空，并且肯定有相关项目。

注意：我只显示了与此问题相关的信息。

我想知道如何在显示相关项目的情况下解决此问题。最好只使用注释。

谢谢。