See, this strikes me as CS-driven code smell. Why not use an association between User and Story (maybe through :assignments) and just `user.stories.create(...)` or `user.assignments.create(:story => story)` ? No spurious class necessary.
That is probably how you would do it in this case, but in more complex scenarios, a simple association wouldn't be sufficient. In those cases, putting the code into a service instead of the controller will make the code easier to test and reuse.
What's an example that exceeds the functionality of associations?
You wouldn't put the code in the controllers; you'd extend the association proxy.