Keep MongoDB connection open while running tests using mocha framework

Question

I'm using something akin to How to properly reuse connection to Mongodb across NodeJs application and modules to keep my mongoDB Connection open.
This requires all the code to go into a MongoDB.connectDB(async (err) => {...} Block.
How would I use this when writing Tests with Mocha.
Do I have to use a separate Connection for every test? Like this?

const MongoDB = require('../src/mongoUtil') // providing access to the mongo database
var events = require('../src/events') // containing all my database Functions

describe('events.js', function () {
  describe('addEvent()', function () {
    it('should return the event, when added succesfully', async function () {
      await MongoDB.connectDB(async (err) => {
        if (err) throw err

        const database = MongoDB.getDB()
        const eventsCollection = database.db().collection('events')

        const event = {
          name: 'test Event',
          members: []
        }

        const result = await events.addEvent(eventsCollection, event)

        MongoDB.disconnectDB()

        if (result.name !== event.name) {
          return new Error('TODO')
        }
      })
    })
  })
})

For this example to work properly I probably also have to manually set the test to done, if I understood the mocha website correctly?
Can I have just one connection?

mongoUtil.js

const MongoClient = require('mongodb').MongoClient
const uri = 'mongodb://localhost:27017/testing'

let _db

const connectDB = async (callback) => {
  try {
    MongoClient.connect(uri, { useNewUrlParser: true }, (err, db) => {
      _db = db
      return callback(err)
    })
  } catch (e) {
    throw e
  }
}

const getDB = () => _db

const disconnectDB = () => _db.close()

module.exports = { connectDB, getDB, disconnectDB }

events.js

const addEvent = async (collection, event) => {
  try {
    const exists = await collection.findOne({ 'name': event.name })
    if (exists) {
      return false
    } else {
      const results = await collection.insertOne(event)
      return results.ops[0]
    }
  } catch (e) {
    throw e
  }
}
module.exports = { addEvent }

*Can I have just one connection?* - no. You may want to do this in `beforeEach` then. — Estus Flask, Aug 23 '18 at 23:53
I thought about that, but that's not possible since everything related to the database has to run inside a `MongoDB.connectDB({...})` Block. — Julian Veerkamp, Aug 24 '18 at 10:24

score 2 · Accepted Answer · answered Aug 24 '18 at 12:43

You don't need to put everything inside MongoDB.connectDB({...}). This is called callback hell which should be avoided when possible. mongoUtil is inconvenient wrapper that doesn't make MongoDB API simpler or easier to use.

await MongoDB.connectDB(async (err) => { ... is a mistake. Putting async function as a callback is not enough and will result in improper control flow.

MongoClient.connect uses error-first callbacks, so it can be used with done to pass errors to test suite:

let db;

beforeEach(done => {
  MongoClient.connect(uri, { useNewUrlParser: true }, (err, _db) => {
    db = _db;
    done(err);
  });
});

connectDB incorrectly promisifies MongoClient.connect. There's no need for one-time callbacks when promises and async..await are in use. It could be:

const connectDB = () => {
  return new Promise((resolve, reject) => {
    MongoClient.connect(uri, { useNewUrlParser: true }, (err, db) => {
      if (err) reject(err);
      else resolve(db);
    });
  });
};

But the fact is that mongodb supports promises natively when callback argument is omitted. mongoUtil needs only one helper function, connectDB:

const connectDB = () => MongoClient.connect(uri, { useNewUrlParser: true });

Which can be used with Mocha seamlessly since it supports promises:

let db;

beforeEach(async () => {
  db = await connectDB();
});

afterEach(() => db.close());

close is asynchronous and returns a promise, too.

This solved my issue, thank you. If I want to use the same solution in app.js, I'd need to wrap it into an async function and use await (like your beforeEach example) or use .then(). Could I just create a `const main = async () => {}` function? — Julian Veerkamp, Aug 24 '18 at 15:14
You're welcome. A common recipe for app entry point now is async IIFE, `(async () => { ... })().catch(console.error)`. — Estus Flask, Aug 24 '18 at 15:29
Hi guys - I am in a similar situation and the solution you are discussing here seems to be closest one to what I want. However, I am having trouble getting it to work. Is it possible to provide a small example that shows both the code in the app.js file and in the test file? I am unable to get the test to exit, since it is not closing the Mongo connection and I don't know how to get the test to close it once it finishes running. I posted about this here - https://stackoverflow.com/questions/63759862/closing-a-mongodb-connection-after-running-mocha-tests-for-an-express-applicatio. — Muthu Palanisamy, Sep 06 '20 at 01:33

Keep MongoDB connection open while running tests using mocha framework

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