How to use mockgoose (or any other db mocking) in express app integration test

I had the same problem as you. In my case, I solved using chai + chai-http and breaking the db connection and app in different files:

db.js:

const mongoose = require('mongoose');const config = require('../../config');mongoose.Promise = global.Promise;mongoose.set('debug', process.env.DEBUG != undefined);function open(){    return new Promise((resolve, reject) => {        if(process.env.DEBUG != undefined) {            let Mockgoose = require('mockgoose').Mockgoose;            let mockgoose = new Mockgoose(mongoose);            mockgoose.helper.setDbVersion("** your mongodb version **");            mockgoose.prepareStorage().then(function() {                mongoose.connect(config.db_test, (err, res) => {                  if (err) return reject(err);                  resolve();                });            }).catch(reject);        }else{            mongoose.connect(config.db, (err, res) => {              if (err) return reject(err);              resolve();            });        }    });}function close(){    return mongoose.disconnect();}module.exports = { close, open };

app.js:

const express = require('express');const bodyParser = require('body-parser');const api = require('./routes');app.use(bodyParser.urlencoded({ extended: false }));app.use(bodyParser.json());app.use('/api', api);module.exports = app;

test.js (for test):

const chai = require('chai');const chaiHttp = require('chai-http');const expect = chai.expect;const conn = require('./../utils/db'); // <-- db.jsconst app = require('../../app'); // <-- app.jschai.use(chaiHttp);describe('# Test', function(){    before(function(done) {        conn.open().then(() => done()).catch(done);    });    after(function(done){        conn.close().then(() => done()).catch(done);    });    it(`test something`, function(done){        chai.request(app) // <-- pass the app here            .get('/path/to/test')            .then((res) => {                // expects                done();              })              .catch((err) => {                  done(err);              });    });});

index.js (for development or production):

const conn = require('./utils/db'); // <-- db.jsconst app = require('./app'); // <-- app.jsconst config = require('./config'); conn.open().then(() => {    app.listen(config.port, () => {        // OK!    });});

I hope it works for you or anyone.