Question

So here’s the routes in question:

  resources :subjects, path: 'library' do
    resources :modules, controller: 'subject_modules'
  end

When I write form_for [@subject, @subject_module] (with those two set to what you’d expect), Rails tries to generate “subject_subject_module_path”.

When I remove the :path for the subjects resource, the generated helper remains the same (as expected).

What it should be is “subject_module_path”; I suspect the problem is that Rails looks at the controller for the modules resources and uses that instead of its actual path, i.e. it builds this helper:

subject_ + subject_module_ + path

From what I’ve gathered so far, it’s pointing towards the possibility of a bug, but is it possible it’s more something on my side or something intended by design?

For now, I suppose this is usable as a temporary fix:

form_for([@subject, @subject_module], url: subject_module_path(@subject, @subject_module))

I’m using Rails 3.1.3.

Answer 1

This is something that occurs due to the design of Rails.

The following is what the Rails API docs say in relation to url_for:

Relying on named routes

Passing a record (like an Active Record or Active Resource) instead of
a Hash as the options parameter will trigger the named route for that
record. The lookup will happen on the name of the class. So passing a
Workshop object will attempt to use the workshop_path route. If you
have a nested route, such as admin_workshop_path you’ll have to call
that explicitly (it’s impossible for url_for to guess that route).

But saying explicitly declaring a helper is required when you have a “nested route” is in accurate. If the nesting occurs under a resource (without certain kinds of routing options, more on this later), Rails will be able to generate the path without any issues.

form_for appears to at some point, like url_for, call polymorphic_url in order to generate the URL to target, which in turn calls build_named_route_call.

You can see that build_named_route_call simply generates underscored_versions of the ModelNames passed in, and joins them together with underscores.

Bringing that back to my routing:

  resources :subjects, path: 'library' do
    resources :modules, controller: 'subject_modules'
  end

Since the subjects resource is under /library/ by setting :path, its helpers remain as subjects_*, and thus Rails has no problems generating a URL for it when passed a Subject. The subject_modules resource (named as such since Rails reserves the name Module for models) however, has had its named helpers changed from its model name through the setting of :controller.

Mystery solved.