Identity Map and Request Context

MikroORM uses identity map in background so you will always get the same instance of one entity.

const authorRepository = orm.em.getRepository(Author);
const jon = await authorRepository.findOne({ name: 'Jon Snow' }, ['books']);
const authors = await authorRepository.findAll(['books']);
// identity map in action
console.log(jon === authors[0]); // true

If you want to clear this identity map cache, you can do so via em.clear() method:

orm.em.clear();

You should always keep unique identity map per each request. This basically means that you need to clone entity manager and use the clone in request context. There are two ways to achieve this:

Forking Entity Manager

With fork() method you can simply get clean entity manager with its own context and identity map:

const em = orm.em.fork();

RequestContext helper for DI containers

If you use dependency injection container like inversify or the one in nestjs framework, it can be hard to achieve this, because you usually want to access your repositories via DI container, but it will always provide you with the same instance, rather than new one for each request.

To solve this, you can use RequestContext helper, that will use node's Domain API in the background to isolate the request context. MikroORM will always use request specific (forked) entity manager if available, so all you need to do is to create new request context preferably as a middleware:

app.use((req, res, next) => {
RequestContext.create(orm.em, next);
});

You should register this middleware as the last one just before request handlers and before any of your custom middleware that is using the ORM. There might be issues when you register it before request processing middleware like queryParser or bodyParser, so definitely register the context after them.

Why is Request Context needed?

Imagine you will use single Identity Map throughout your application. It will be shared across all request handlers, that can possibly run in parallel.

Problem 1 - growing memory footprint

As there would be only one shared Identity Map, you can't just clear it after your request ends. There can be another request working with it so clearing the Identity Map from one request could break other requests running in parallel. This will result in growing memory footprint, as every entity that became managed at some point in time would be kept in the Identity Map.

Problem 2 - unstable response of API endpoints

Every entity has toJSON() method, that automatically converts it to serialized form. If you have only one shared Identity Map, following situation may occur:

Let's say there are 2 endpoints

  1. GET /book/:id that returns just the book, without populating anything
  2. GET /book-with-author/:id that returns the book and its author populated

Now when someone requests same book via both of those endpoints, you could end up with both returning the same output:

  1. GET /book/1 returns Book without populating its property author property
  2. GET /book-with-author/1 returns Book, this time with author populated
  3. GET /book/1 returns Book, but this time also with author populated

This happens because the information about entity association being populated is stored in the Identity Map.