MikroORM uses identity map in background so you will always get the same instance of
If you want to clear this identity map cache, you can do so via
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
fork() method you can simply get clean entity manager with its own context and identity map:
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:
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
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
GET /book/:idthat returns just the book, without populating anything
GET /book-with-author/:idthat 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:
Bookwithout populating its property
Book, this time with
Book, but this time also with
This happens because the information about entity association being populated is stored in the Identity Map.