如何获取已保存文件的ID？

Question

我有一个nodeJS服务器，它每当接收到文件时就使用multer将文件保存在MongoDB数据库中，我还想保存另一个包含已保存文件ID的文档，但是如何获取它ID ？

const conn = mongoose.createConnection(mongoURI);

conn.once("open", () => {
  // Init stream
  gfs = Grid(conn.db, mongoose.mongo);
  gfs.collection("ProfilePic");
});

// Create storage engine
const storage = new GridFsStorage({
  url: mongoURI,
  file: (req, file) => {
    return new Promise((resolve, reject) => {
      const filename = file.originalname;
      const fileInfo = {
        filename: filename,
        bucketName: "ProfilePic"
      };
      resolve(fileInfo);
    });
  }
});

const upload = multer({ storage });

router.post("/setProfilePic", upload.single("picture"), (req, res) => {
  console.log("image received");
  let possederProfilePic = await new PossederProfilePic({
    _idUser = req.body.userID,
    _idProfilePic = IdOfTheFileSavedPrevioulsy
  })
  possederProfilePic.save()
  .then(user => {
    res.status(201).json({ error: false });
  })
  .catch(err => {
    res.status.json({ error: err });
  });
});