Symfony 2，从多对多关联中获取实体

我有实体Product和Tires。一个产品有多个轮胎，一个轮胎可以在多个产品上。

$product = $em->getRepository('MyBundle:Product')->find($id);

或

$tires = $product->getExtraTires();

Related Tires won't return.

/**
 * Product
 *
 * @ORM'Table(name="product")
 * @ORM'Entity
 */
class Product {
    public function __construct()
    {
        $this->extraTires = new ArrayCollection();
    }
   /**
     * @ORM'ManyToMany(targetEntity="Tires", mappedBy="products")
     * @ORM'JoinTable(name="product_tires",
     *       joinColumns={@ORM'JoinColumn(name="product_id", referencedColumnName="id")},
     *       inverseJoinColumns={@ORM'JoinColumn(name="tires_id", referencedColumnName="id")})
     */
    private $extraTires;
   /**
     * Get extraTires
     *
     * @return ArrayCollection
     */
     public function getExtraTires()
     {
         return $this->extraTires;
     }
    /**
      * Add extraTires
      *
      * @param Tires $extraTires
      * @return Product
      */
      public function addExtraTire(Tires $extraTires)
      {
          $this->extraTires[] = $extraTires;
          return $this;
      }
     /**
      * Remove extraTires
      *
      * @param 'Wielton'WieltonBundle'Entity'Tires $extraTires
      */
      public function removeExtraTire(Tires $extraTires)
      {
          $this->extraTires->removeElement($extraTires);
      }

和我的轮胎实体

/**
 * Tires
 *
 * @ORM'Table(name="Tires")
 * @ORM'Entity
 */    
class Tires {
    public function __construct()
    {
        $this->products = new ArrayCollection();
    }
    /**
     * @var integer
     *
     * @ORM'Column(name="id", type="integer")
     * @ORM'Id
     * @ORM'GeneratedValue(strategy="AUTO")
     */
     private $id;
    /**
     * @ORM'ManyToMany(targetEntity="Product", inversedBy="extraTires")
     * @ORM'JoinTable(name="product_tires",
     *       joinColumns={@ORM'JoinColumn(name="tires_id", referencedColumnName="id")},
     *       inverseJoinColumns={@ORM'JoinColumn(name="product_id", referencedColumnName="id")})
     */
     private $products;
    /**
     * Add products
     *
     * @param Product $products
     * @return Tires
     */
     public function addProduct(Product $products)
    {
        $this->products[] = $products;
        return $this;
    }
    /**
     * Remove products
     *
     * @param Product $products
     */
     public function removeProduct(Product $products)
    {
        $this->products->removeElement($products);
    }
    /**
     * Get products
     *
     * @return 'Doctrine'Common'Collections'Collection 
     */
     public function getProducts()
     {
        return $this->products;
     }

我根据理论文档设计实体。当我添加Product并选择轮胎添加时，在连接表中看起来一切正常。

我错过了什么?

编辑

我发现了一些东西，但我不完全明白它是如何工作的:)

在我的控制器，当我得到产品实体与

$product = $em->getRepository('MyBundle:Product')->find($id);

我得进去了product.extraTires.owner.extraTires然后我可以在这里看到相关的轮胎。突然间，我又有一个对数据库的查询。所以我没有在控制器中正确调用它

$product = $em->getRepository('MyBundle:Product')->find($id);
$tires = $product->getExtraTires()->toArray();