如何让Gedmo\Timestampable在第一次创建时保持updated_at为空？

use Gedmo\Mapping\Annotation as Gedmo;

/**
 * @ORM\Entity
 */
class YourEntity
{
    // ...

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

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

    // ...
}

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity
 */
class YourEntity
{
    // ...

    public function __construct()
    {
        $this->updatedAt = null;
    }

    // ...
}