Hi!
The problem: You may have read the I18n-Guide (http://guides.rubyonrails.org/ i18n.html#setting-the-locale-from-the-url-params). It says, that the usual way to provide the locale is in the URL, like example.com/en/ books/2. This route would look like:
map.resources :books, :path_prefix => '/:locale'
[...]
This solution has currently one rather big *downside*. Due to the _default_url_options_ implementation, you have to pass the :id option explicitly, like this:
link_to 'Show', book_url(:id => book)
and not depend on Rails’ magic in code like
link_to 'Show', book.
Actually this is not a problem of default_url_options, but of polymorphic_url, which is utilized by link_to in the example. It generates the book_url and passes an array into the generated route. This array has only one item, the book id. book_url tries to fill in the id into the first parameter of the url, which is the locale (/:locale/books/:id), which leaves the :id unsatisfied, raising an exception.
The solution: With my patch, polymorphic_url will pass an hash instead of an array to the generated route. It may look like {:id => 2}. It works also with nested resources: e.g.: {:author_id => 3, :id => 2}
The patch: I've created a patch for Rails 3 and Rails 2-3-stable and put them in a ticked on lighthouse: https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/3394-polymorphic_url-fails-to-generate-routes-with-path_prefix-with-a-variable
I hope, you'll find this patch useful and it gets accepted into rails soon.
Regards from Hamburg, Germany
Johannes PS: The guide how to test patches is here: https://rails.lighthouseapp.com/projects/8994/sending-patches
