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 EntityManager.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 in middle:

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.