how to use the connection.db.collection function? - node.js

Question

I have implemented the following code from this link:
What is best way to handle global connection of Mongodb in NodeJs
to create a class for the connection of MongoDB. But when I try to call the singleton class in the router, I get the following error:
TypeError: Connection.db.collection is not a function
mongodb.js
const MongoClient = require('mongodb').MongoClient
const url = '...';
class Connection {
static connectToDB() {
if ( this.database ) return Promise.resolve(this.database)
return MongoClient.connect(this.url, {useNewUrlParser: true}, (err, db) => {
if (err) console.log(err);
else {
this.db = db;
console.log('MongoDB connected');
}
})
}
}
Connection.db = null
Connection.url = url
Connection.options = {
bufferMaxEntries: 0,
reconnectTries: 5000,
}
module.exports = Connection;
app.js
const express = require('express');
const app = express();
let bodyParser = require('body-parser')
// mongodb config
const Connection = require('../config/mongodb');
const server = app.listen(3000, () => {
Connection.connectToDB(); // output: MongoDB connected
console.log(`listening to port: ${port} on http://127.0.0.1:3000}/`); // output: listening to port: 3000 on http://127.0.0.1:3000/
});
router.js
const router = require('express').Router();
let Connection = require('../config/mongodb');
router.post('/data', (req, res) => {
Connection.db.collection('tweets').find({}) // output: TypeError: Connection.db.collection is not a function
.then(tweets => console.log(tweets))
.catch(err => console.log(err));
});

Answer 1

Try once to package.json, change mongodb line to "mongodb": "^2.2.33". You will need to npm uninstall mongodb; then npm install to install this version.

Answer 2

The question you linked uses promises throughout, whereas you're using the callback version of connect.
return MongoClient.connect(this.url, {useNewUrlParser: true}, (err, db) => ...
You then call this without returning in your server:
Connection.connectToDB();
console.log(`listening to port: ${port} on http://127.0.0.1:3000}/`);
There is therefore no guarantee that your connection will have been made by the time your first api request comes in. In fact, if you did:
Connection.connectToDB();
console.log(`listening to port: ${port} on http://127.0.0.1:3000}/`);
Connection.db.collection('tweets').find({});
It would fail every time as Connection.db will still be null.
In the example you linked, using Promises protect against that. Note in particular the connect method:
static connectToDB() {
if ( this.database ) return Promise.resolve(this.database)
// ** USING THE PROMISE VERSION OF CONNECT **
return MongoClient.connect(this.url, this.options)
.then(db => this.db = db)
}
and your usage code should also use promises:
return Connection.connectToDB()
.then(() => {
// do something here
});