How to set defaults for parameters of url helper methods?

Question

I use language code as a prefix, e.g. www.mydomain.com/en/posts/1. This is what I did in routes.rb:

scope ":lang" do
  resources :posts
end

Now I can easily use url helpers such as: post_path(post.id, :lang => :en). The problem is that I would like to use a value in a cookie as a default language. So I could write just post_path(post.id).

Is there any way how to set default values for parameters in url helpers? I can't find the source code of url helpers - can someone point me in the right direction?

Another way: I have already tried to set it in routes.rb but it's evaluated in startup time only, this does not work for me:

scope ":lang", :defaults => { :lang => lambda { "en" } } do
  resources :posts
end

Answer 1

Ryan Bates covered this in todays railscast: http://railscasts.com/episodes/138-i18n-revised

You find the source for url_for here: http://api.rubyonrails.org/classes/ActionDispatch/Routing/UrlFor.html

You will see it merges the given options with url_options, which in turn calls default_url_options.

Add the following as private methods to your application_controller.rb and you should be set.

def locale_from_cookie
  # retrieve the locale
end

def default_url_options(options = {})
  {:lang => locale_from_cookie}
end