Doctrine Extensions Symfony2

Question

我安装了像How to install Doctrine Extensions in a Symfony2 project

这样的Doctrine Extensions

这是我的实体类

<?php

namespace My\NewsBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Gedmo\Mapping\Annotation as Gedmo;

/**
 * News
 *
 * @ORM\Table()
 * @ORM\Entity
 */
class News
{
    /**
     * @var integer
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;

    /**
     * @ORM\Column(type="string", length=150)
     */
    protected $title;

    /**
     * @ORM\Column(type="text")
     */
    protected $content;

    /**
     * @ORM\Column(type="text")
     */
    protected $description;

    /**
     * @ORM\Column(type="string", length=150)
     */
    protected $keywords;

    /**
     * @Gedmo\Slug(fields={"title"})
     * @ORM\Column(length=128, unique=true)
     */
    private $slug;

    /**
     * @Gedmo\Timestampable(on="create")
     * @ORM\Column(name="created_at", type="datetime")
     */
    private $created_at;

    /**
     * @ORM\Column(name="updated_at", type="datetime")
     * @Gedmo\Timestampable(on="update")
     */
    private $updated_at;



    /**
     * Get id
     *
     * @return integer 
     */
    public function getId()
    {
        return $this->id;
    }
}

php app/console doctrine:generate:entities My

给我一​​个错误

[Doctrine\Common\Annotations\AnnotationException]                            
  [Semantical Error] The annotation "@Gedmo\Mapping\Annotation\Slug" in property My\NewsBundle\Entity\News::$slug does not exist, or could not be auto -loaded.

我应该在配置中添加一些内容还是其他内容？谢谢你的帮助。