docs: add jest-mongodb example

docs/MongoDB.md

@@ -0,0 +1,126 @@
+---
+id: mongodb
+title: Using with MongoDB
+---
+
+With the [Global Setup/Teardown](Configuration.md#globalsetup-string) and
+[Async Test Environment](Configuration.md#testenvironment-string) APIs, Jest can
+work smoothly with [MongoDB](https://www.mongodb.com/).
+
+## 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
+
+```js
+// setup.js
+const MongodbMemoryServer = require('mongodb-memory-server');
+
+const MONGO_DB_NAME = 'jest';
+const mongod = new MongodbMemoryServer.default({
+  instance: {
+    dbName: MONGO_DB_NAME
+  },
+  binary: {
+    version: '3.2.19'
+  }
+});
+
+module.exports = function() {
+  global.__MONGOD__ = mongod;
+  global.__MONGO_DB_NAME__ = MONGO_DB_NAME;
+};
+```
+
+Then we need a custom Test Environment for puppeteer
+
+```js
+// mongo-environment.js
+class MongoEnvironment extends NodeEnvironment {
+  constructor(config) {
+    super(config);
+  }
+
+  async setup() {
+    console.log('Setup MongoDB Test Environment');
+
+    this.global.__MONGO_URI__ = await global.__MONGOD__.getConnectionString();
+    this.global.__MONGO_DB_NAME__ = global.__MONGO_DB_NAME__;
+
+    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
+
+```js
+// teardown.js
+module.exports = async function() {
+  await global.__MONGOD__.stop();
+};
+```
+
+With all the things set up, we can now write our tests like this:
+
+```js
+// 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 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}
+  ]);
+});
+```
+
+Note: you may need to modify test script to run jest sequentially, like that:
+`jest --runInBand --forceExit`.
+
+Here's the code of
+[full working example](https://github.com/vladgolubev/jest-mongodb).

website/sidebars.json

@@ -17,6 +17,7 @@
       "es6-class-mocks",
       "webpack",
       "puppeteer",
+      "mongodb",
       "migration-guide",
       "troubleshooting"
     ],