Basic CS -> Rails routes converter.

coffee_script_route_parser.rb

require 'rails/application/route_inspector'

class Journey::Visitors::Formatter
  def visit_SYMBOL node
    key = node.to_sym

    if value = options[key]
      consumed[key] = value
      value # Overritten this line to avoid escaping of characters
    else
      "\0"
    end
  end
end

class CoffeeScriptRouteParser
  def self.parse(name, *unnamed_params)
    
    # Split params for product_option_path(:product_id, :id, {format: 'json', anchor: 'foobar'})
    # into [:product_id, :id], and {format...}
    named_params = unnamed_params.last.is_a?(Hash) ? unnamed_params.pop : {}
    
    name.gsub!(/_path$/, '')
    name.gsub!(/_url$/, '')
    
    # Get the relevant route.
    route = Rails.application.routes.routes.select {|route| route.name == name }.first()
    
    # Convert parts into a hash
    parts = {}
    
    # Override any named params
    named_params.each do |k,v| 
      parts[k] = '#{' + v + '}'
    end
    
    # And any unnamed params
    unnamed_params.each_with_index do |param, index|
      parts[route.parts[index]] = '#{' + param + '}'
    end
    
    # Now use the overriden formatter to create the URL.
    route.format(parts)
  end
end

routes.rb

RailsCoffeescriptRoutes::Application.routes.draw do
  
  resources :products do 
    resources :options
  end
  match 'products/:id' => 'catalog#view', as: :catalog
  match 'products/:id/purchase' => 'catalog#purchase', :as => :purchase

  match ':controller(/:action(/:id))(.:format)', as: :lots_of_options
end

routing_test.rb

require 'test_helper'

class RoutingTest < ActiveSupport::TestCase
  test "truth" do
    assert_kind_of Module, RailsCoffeescriptRoutes
    Rails.application.routes.routes.each{|route|p route.name if route.name}
  end
  
  ####
  #
  # This sits as part of coffee-rails
  # It adds a list of reserved methods to CoffeeScript with the names of the routes.
  # When CS hits a reserved method name, it then backs it off to a function such as CoffeeScriptRouteParser.parse
  # which takes the method name and the arguments. I'm presuming coffeescript is tokenising the code.
  # If not, then we'll need more work to get it out of CS.
  #
  # It then simply uses the Rails routing internals to get the proper URL to call.
  #
  # This currently outputs as coffeescript using #{} for string concatenation.
  # It might be better to use Javasacript to do it, but I know very little about the CS compiler.
  #----
  test "resources paths" do
    # products_path
    assert_equal '/products', CoffeeScriptRouteParser.parse('products_path')
    # products_path(format: 'json')
    assert_equal '/products.#{"json"}', CoffeeScriptRouteParser.parse('products_path', format: '"json"')
    
    # product_path(1)
    assert_equal '/products/#{"1"}', CoffeeScriptRouteParser.parse('product_path', '"1"')
    # product_path(1, format: json)
    assert_equal '/products/#{"1"}.#{"json"}', CoffeeScriptRouteParser.parse('product_path', '"1"', format: '"json"')
    
    assert_equal '/products/new', CoffeeScriptRouteParser.parse('new_product_path')
    assert_equal '/products/edit', CoffeeScriptRouteParser.parse('edit_product_path')
  end
  
  test "nested resources paths" do
    assert_equal '/products/#{product_id}/options/#{option_id}', CoffeeScriptRouteParser.parse('product_option_path', 'product_id', 'option_id')
  end
  
  test "manual paths" do
    assert_equal '/products/#{some_var}', CoffeeScriptRouteParser.parse('catalog_path', "some_var")
    assert_equal '/products/#{some_var}/purchase', CoffeeScriptRouteParser.parse('purchase_path', "some_var")
  end
  
  test "lots of options" do
    assert_equal '', CoffeeScriptRouteParser.parse('lots_of_options')
    assert_equal '/#{controller_var}', CoffeeScriptRouteParser.parse('lots_of_options', controller: 'controller_var')
  end
end

There's a repository with these files in at https://github.com/ihid/rails-coffeescript-routes - if you want to send pull-requests there with failing tests, then I will get them passing.