As per the documentation on http://guides.rubyonrails.org/configuring.html#deploy-to-a-subdirectory-relative-url-root I set the ENV var RAILS_RELATIVE_URL_ROOT = "/app1".

I setup a basic route:

Rails.application.routes.draw do
  root 'pages#home'
  resources :cats, only: [:show, :index]
end

To test this I output:

# views/pages/home.html.erb
<pre>
  <%= cats_url %>
  <%= cats_path %>
  <%= Rails.application.config.relative_url_root %>
</pre>

The output is:

  http://localhost:3000/cats
  /cats
  /app1

Not http://localhost:3000/app1/cats as could be expected.

Sorry if there is no test to go along with this issue but I currently have really limited bandwidth and cannot download the whole rails repo to set it up.