mongoose - multer - multer-gridfs-storage: The database connection must be open to store files

Question

I had this error while I am trying to use multer-gridfs-storage, I want to upload files and store them in mongoDB atlas

index.js file:

require("dotenv").config();
const upload = require("./routes/upload");`your text`
const Grid = require("gridfs-stream");
const mongoose = require("mongoose");
const connection = require("./db");
const express = require("express");
const app = express();

let gfs;
connection();

const conn = mongoose.connection;
conn.once("open", function () {
  gfs = Grid(conn.db, mongoose.mongo);
  gfs.collection("photos");
});

app.use("/file", upload);

// media routes
app.get("/file/:filename", async (req, res) => {
  try {
    const file = await gfs.files.findOne({ filename: req.params.filename });
    const readStream = gfs.createReadStream(file.filename);
    readStream.pipe(res);
  } catch (error) {
    res.send("not found");
  }
});

app.delete("/file/:filename", async (req, res) => {
  try {
    await gfs.files.deleteOne({ filename: req.params.filename });
    res.send("success");
  } catch (error) {
    console.log(error);
    res.send("An error occured.");
  }
});

const port = process.env.PORT || 5666;
app.listen(5000, console.log(`Listening on port ${port}...`));

db.js file

const mongoose = require("mongoose");
module.exports = async function connection() {
  try {
    await mongoose.connect(
      "mongodb+srv://gmcws2024:[email protected]/camping-app?retryWrites=true&w=majority"
    );
    console.log("connected to database");
  } catch (error) {
    console.log(error);
    console.log("could not connect to database");
  }
};`

**upload.js middleware**
`const multer = require("multer");
const  GridFsStorage  = require("multer-gridfs-storage");

const storage = new GridFsStorage({
  url: "mongodb+srv://gmcws2024:[email protected]/camping-app?retryWrites=true&w=majority",
  file: (req, file) => {
    const match = ["image/png", "image/jpeg"];
    if (match.indexOf(file.mimetype) === -1) {
      const filename = `${Date.now()}-any-name-${file.originalname}`;
      return filename;
    }
    return {
      bucketName: "photos",
      filename: `${Date.now()}-any-name-${file.originalname}`,
    };
  },
});

module.exports = multer({ storage });

upload route

const upload = require("../middleware/upload");
const express = require("express");
const router = express.Router();

router.post("/upload", upload.single("file"), async (req, res) => {
  if (req.file === undefined) return res.send("you must select a file.");
  const imgUrl = `${req.protocol}://${req.headers.host}/file/${req.file.filename}`;
  return res.send(imgUrl);
});

module.exports = router;

package.json file

{
  "name": "nouveau-dossier",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "dev": "nodemon index.js"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "dotenv": "^16.3.1",
    "express": "^4.18.2",
    "gridfs-stream": "^1.1.1",
    "mongoose": "^8.0.3",
    "multer": "^1.4.4",
    "multer-gridfs-storage": "^4.2.0",
    "nodemon": "^3.0.2"
  }
}

but this error accured : "Error: The database connection must be open to store files at GridFSStorage._handleFile (C:\Users\MOHAMED ANWAR\Desktop\Nouveau dossier\node_modules\multer-gridfs-storage\lib\gridfs.js:321:6) at C:\Users\MOHAMED ANWAR\Desktop\Nouveau dossier\node_modules\multer\lib\make-middleware.js:145:17
at allowAll (C:\Users\MOHAMED ANWAR\Desktop\Nouveau dossier\node_modules\multer\index.js:8:3) at wrappedFileFilter (C:\Users\MOHAMED ANWAR\Desktop\Nouveau dossier\node_modules\multer\index.js:44:7)
at Busboy. (C:\Users\MOHAMED ANWAR\Desktop\Nouveau dossier\node_modules\multer\lib\make-middleware.js:115:7) at Busboy.emit (node:events:514:28) at Busboy.emit (C:\Users\MOHAMED ANWAR\Desktop\Nouveau dossier\node_modules\busboy\lib\main.js:38:33)
at PartStream. (C:\Users\MOHAMED ANWAR\Desktop\Nouveau dossier\node_modules\busboy\lib\types\multipart.js:213:13) at PartStream.emit (node:events:514:28) at HeaderParser. (C:\Users\MOHAMED ANWAR\Desktop\Nouveau dossier\node_modules\dicer\lib\Dicer.js:51:16)"