application_controller.rb

 def default_url_options(options = {})
   # resource hack so that url_for(@post) works like it should
   # if a post was given as a parameter to `post_path` it will be assigned to the first (unnassigned) parameter of the route (year)
   if options[:controller] == "posts" && options[:year].is_a?(Post)
     post = options[:year]
     options[:year] = post.year
     options.merge(:month => post.month, :slug => post.slug)
   else
     {}
   end
 end