MongooseError: You can not `mongoose.connect()` multiple times while connected

In order to use multiple MongoDB connections use mongoose.createConnection function instead of mongoose.connect.

mongoose.createConnection Will give you a connection object which you can further use in your model file, Cause models are always bound to a single connection

let config = require('../config');
let mongoose = require('mongoose');

exports.connect = function () {
  const db = mongoose.createConnection(config.mongoUrl, {
    reconnectInterval: 5000,
    reconnectTries: 60
    // add more config if you need
  });
  db.on(`error`, console.error.bind(console, `connection error:`));
  db.once(`open`, function () {
    // we`re connected!
    console.log(`MongoDB connected on "  ${config.mongoUrl}`);
  });
};

I've had the same problem and solved pretty easily. All i had to do was to remove any connections in my controllers.

Before: Server.js

const mongoose = require('mongoose');
const connectionString = 'mongodb://localhost:27017/DB';
mongoose.connect(connectionString);
const db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
//Server code...

Controller.js

const mongoose = require('mongoose');
const connectionString = 'mongodb://localhost:27017/DB';
mongoose.connect(connectionString);
const db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
//Controller code...

After: Server.js

const mongoose = require('mongoose');
const connectionString = 'mongodb://localhost:27017/DB';
mongoose.connect(connectionString);
const db = mongoose.connection;
db.on('error', console.error.bind(console, 'connection error:'));
//Server code...

Controller.js

//Controller code...

Obviously I removed it from all my controller files.

In mongoose version 5.6.1 the check was added https://github.com/Automattic/mongoose/pull/7905

Revert to an older version for a quick fix.