Write links in Rails 3 with optional path prefix

TL;DR: how can I have Rails 3 route without a path prefix, but write URLs with the prefix?

Let's say you have a rails 3 app with a controller for bars. You deploy it to your WebFaction website with a URL path of /foo. To get a list of bars, you go to http://host.com/foo/bars. The web server routes the path /foo to passenger, and when the request gets to rails' routing engine, it looks like a request for /bars.

Now you want to link to your list of bars, so you write link_to bar_path. For the link to be routed on this host, it needs to be written out as /foo/bars. In fact, links to any controller in this app must be prefixed with "/foo".

In Rails 3, this is achievable using a scope:

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  scope 'foo' do
    resources :bars
  end
end

Now link_to bar_path will write /foo/bars. Unfortunately, that resource now lives at /foo/foo/bars, because the first /foo is consumed routing the request to Passenger. You can fix this by making the scope optional:

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  scope '(foo)' do
    resources :bars
  end
end

Now both /foo/foo/bars and /foo/bars will route to BarsController. Unfortunately, link_to bar_path now writes /bars, so the request will never be routed to our rails app in the first place.

Is there a way to write the link_to so that it includes the optional scope? Or is there a simpler way to handle this setup? I know I can just duplicate all the routes with and without the scope, but that isn't very DRY:

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  scope 'foo' do
    resources :bars
  end

  resources :bars
end