url_for with subdomains

class ApplicationController < ActionController::Base

  private
  
  # To write subdomains on the url helpers:
  # root_url(nil, {:subdomain => "subdomain"})
  def url_for(options = nil)
    case options
    when Hash
      if subdomain = options.delete(:subdomain)
        if request.subdomain.empty?
          options[:host] = "#{subdomain}.#{request.host_with_port}"
        else
          options[:host] = request.host_with_port.sub(request.subdomain, subdomain)
        end
      end
    end
    super
  end
end