With the Global Setup/Teardown and Async Test Environment APIs, Jest can work smoothly with MongoDB.

A jest-mongodb example

The basic idea is to:

1. Spin up in-memory mongodb server
2. Export a global variable with mongo URI
3. Write tests for queries / aggregations using a real database ✨
4. Shut down mongodb server using Global Teardown

Here's an example of the GlobalSetup script

// setup.js
const path = require('path');

const fs = require('fs');

const MongodbMemoryServer = require('mongodb-memory-server');

const globalConfigPath = path.join(__dirname, 'globalConfig.json');

const mongoServer = new MongodbMemoryServer.MongoMemoryServer();

module.exports = async function() {
  const mongoConfig = {
    mongoDBName: 'jest',
    mongoUri: await mongoServer.getConnectionString(),
  };

  // Write global config to disk because all tests run in different contexts.
  fs.writeFileSync(globalConfigPath, JSON.stringify(mongoConfig));

  // Set reference to mongod in order to close the server during teardown.
  global.__MONGOD__ = mongoServer;
};

Then we need a custom Test Environment for Mongo

// mongo-environment.js
const NodeEnvironment = require('jest-environment-node');

const path = require('path');

const fs = require('fs');

const globalConfigPath = path.join(__dirname, 'globalConfig.json');

class MongoEnvironment extends NodeEnvironment {
  constructor(config) {
    super(config);
  }

  async setup() {
    console.log('Setup MongoDB Test Environment');

    const globalConfig = JSON.parse(fs.readFileSync(globalConfigPath, 'utf-8'));

    this.global.__MONGO_URI__ = globalConfig.mongoUri;
    this.global.__MONGO_DB_NAME__ = globalConfig.mongoDBName;

    await super.setup();
  }

  async teardown() {
    console.log('Teardown MongoDB Test Environment');

    await super.teardown();
  }

  runScript(script) {
    return super.runScript(script);
  }
}

Finally we can shut down mongodb server

// teardown.js
module.exports = async function() {
  await global.__MONGOD__.stop();
};

With all the things set up, we can now write our tests like this:

// test.js
const {MongoClient} = require('mongodb');

let connection;
let db;

beforeAll(async () => {
  connection = await MongoClient.connect(global.__MONGO_URI__);
  db = await connection.db(global.__MONGO_DB_NAME__);
});

afterAll(async () => {
  await connection.close();
  await db.close();
});

it('should aggregate docs from collection', async () => {
  const files = db.collection('files');

  await files.insertMany([
    {type: 'Document'},
    {type: 'Video'},
    {type: 'Image'},
    {type: 'Document'},
    {type: 'Image'},
    {type: 'Document'},
  ]);

  const topFiles = await files
    .aggregate([
      {$group: {_id: '$type', count: {$sum: 1}}},
      {$sort: {count: -1}},
    ])
    .toArray();

  expect(topFiles).toEqual([
    {_id: 'Document', count: 3},
    {_id: 'Image', count: 2},
    {_id: 'Video', count: 1},
  ]);
});

Here's the code of full working example.