被Symfony上传文档困惑

Question

我正在尝试关注Symfony 2.1 documentation for handling uploads，我对其代码中的某个变量来自何处感到困惑。

$ this-＆gt;文件变量来自哪里上传方法？

Symfony上传方法：

public function upload()
{
    // the file property can be empty if the field is not required
    if (null === $this->file) {
        return;
    }

    // use the original file name here but you should
    // sanitize it at least to avoid any security issues

    // move takes the target directory and then the
    // target filename to move to
    $this->file->move(
        $this->getUploadRootDir(),
        $this->file->getClientOriginalName()
    );

   // set the path property to the filename where you've saved the file
    $this->path = $this->file->getClientOriginalName();

    // clean up the file property as you won't need it anymore
    $this->file = null;
}

因为文档前面描述的Document实体只有三个变量，即id，name和path

Symfony实体：

 /**
* @ORM\Entity
*/
class Document
{
    /**
     * @ORM\Id
     * @ORM\Column(type="integer")
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    public $id;

    /**
     * @ORM\Column(type="string", length=255)
     * @Assert\NotBlank
     */
    public $name;

    /**
     * @ORM\Column(type="string", length=255, nullable=true)
     */
    public $path;
}

我误解了吗？