How to download a file using gridFs

Question

I was able to upload a file to my mongoDB doing this:

uploadFile(file, uid) {
    var fs = require('fs');
    mongoose.connect(config.db, {useNewUrlParser: true},).catch(e => console.log(e));
    var conn = mongoose.connection;
    grid.mongo = mongoose.mongo;
    const gfs = grid(conn.db);
    const writeStream = gfs.createWriteStream({
        filename: file.filename,
    });
    fs.createReadStream(file.path).pipe(writeStream);
    writeStream.on('close', file => {
        const {name} = file;
        console.log(`${name} written to the db`);
        return Account.findByIdAndUpdate(uid, {'employer.logo': name}).then(r => console.log(r)).catch(e => console.log(e));
    });
}

I found this questions, which helped a bit, but I am still not sure about a couple of things.

The code suggested in the answer is this:

var conn = mongoose.connection;
var gfs = Grid(conn.db, mongoose.mongo);
gfs.findOne({_id: req.params.ID, root: 'resume'}, function (err, file) {
    if (err) {
        return res.status(400).send(err);
    }
    else if (!file) {
        return res.status(404).send('Error on the database looking for the file.');
    }

    res.set('Content-Type', file.contentType);
    res.set('Content-Disposition', 'attachment; filename="' + file.filename + '"');

    var readstream = gfs.createReadStream({
        _id: req.params.ID,
        root: 'resume'
    });

    readstream.on("error", function (err) {
        res.end();
    });
    readstream.pipe(res);
});

What is the "root" part? Am I even using the correct thing? The files are saved in fs.files and fs.chunks.

I tried to convert the code to my case. This is what I got:

downloadFile: async (req, res) => {
    var mongoose = require('mongoose');
    var Grid = require('gridfs-stream');

    mongoose.connect(config.db, {useNewUrlParser: true},).catch(e => console.log(e));
    var conn = mongoose.connection;
    conn.once('open', function () {
        var gfs = Grid(conn.db, mongoose.mongo);
        gfs.findOne({_id: req.params.logoId}, function (err, file) {
            if (err) {
                console.log(err);
                return res.status(400).send(err);
            }
            else if (!file) {
                console.log('no file');
                return res.status(404).send('Error on the database looking for the file.');
            }
            res.set('Content-Type', file.contentType);
            res.set('Content-Disposition', 'attachment; filename="' + file.filename + '"');
            var readstream = gfs.createReadStream({
                _id: req.params.ID,
            });
            readstream.on("error", function (err) {
                console.log(err);
                res.end();
            });
            readstream.pipe(res);
        });
        // all set!
    })
}

But now I'm getting:

TypeError: cursor.nextObject is not a function

at /home/alex/Documents/Projects/ontario-job-portal/node_modules/gridfs-stream/lib/index.js:143:12

at handleCallback (/home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/node_modules/mongodb/lib/utils.js:128:55)

at Collection.<anonymous> (/home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/node_modules/mongodb/lib/collection.js:431:45)

at Collection.deprecated [as find] (/home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/node_modules/mongodb/lib/utils.js:661:17)

at Grid.findOne (/home/alex/Documents/Projects/ontario-job-portal/node_modules/gridfs-stream/lib/index.js:140:14)

at NativeConnection.<anonymous> (/home/alex/Documents/Projects/ontario-job-portal/globals/functions.js:114:17)

at Object.onceWrapper (events.js:313:30)
    at emitNone (events.js:106:13)
    at NativeConnection.emit (events.js:208:7)
    at /home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/lib/connection.js:567:13

at result (/home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/node_modules/mongodb/lib/utils.js:414:17)

at executeCallback (/home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/node_modules/mongodb/lib/utils.js:406:9)

at err (/home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/node_modules/mongodb/lib/operations/mongo_client_ops.js:286:5)

at connectCallback (/home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/node_modules/mongodb/lib/operations/mongo_client_ops.js:241:5)

at process.nextTick (/home/alex/Documents/Projects/ontario-job-portal/node_modules/mongoose/node_modules/mongodb/lib/operations/mongo_client_ops.js:463:7)

at _combinedTickCallback (internal/process/next_tick.js:131:7)

I was using this link to try and make it work https://github.com/aheckmann/gridfs-stream#accessing-file-metadata