What is the preferred way to inject code into other module resolvers?

[EloB]

In this example you have a Core module that contains the type User that have linked account to different platforms.

Is there any best practices on how to extend the loaders inside the platform resolver depending on what modules being registered? I would like to keep all platform related code in separate modules but I don't really know how to write that code. I've already have some code that is working but it more or less an "hack" and it's not aware which modules being registered by the application. I tried to use middleware but I didn't get it to work and my understanding is that middleware are not being merged/concatinated only picking the first module that defines a specific path? If they were merged it would be possible to achieve what I'm looking for. Maybe I've missed something or isn't this supported at the moment?

This is a very simple example and only two platforms but a more advanced example might involve hundreds of platforms.

Here are a simple codesandbox with my preferred way (not working because middlewares not being merged):
https://codesandbox.io/s/graphql-module-feature-request-middlewares-merge-kuq4uf?file=/index.js

Here are the playground

Dynamic fetch Facebook:
https://kuq4uf.sse.codesandbox.io/?query=query+getUserFacebook+%7B%0A++user%28id%3A+1%29+%7B%0A++++platform%28name%3A+FACEBOOK%29+%7B%0A++++++__typename%0A++++++username%0A++++%7D%0A++%7D%0A%7D

Dynamic fetch Google:
https://kuq4uf.sse.codesandbox.io/?query=query+getUserGoogle+%7B%0A++user%28id%3A+1%29+%7B%0A++++platform%28name%3A+GOOGLE%29+%7B%0A++++++__typename%0A++++++username%0A++++%7D%0A++%7D%0A%7D

I would like to be able to write queries like this:

query getUser($id: ID!, $platform: PlatformName) {
  user(id: $id) {
    facebook {
      username
    }
    platform(platform: $platform) {
      username
    }
  }
}