Generating fancy URLs with Rails

There’s a problem with Rails resourceful routes. It is very easy to generate standard resourceful routes; it is equally easy to match incoming fancy URLs. But DRYly generating fancy URLs is a lot harder than suspected.

Matching fancy incoming routes is easy enough:

get '/posts/:year/:month/:id' => 'posts#show'

Our application will now correctly match URLs like /posts/2011/3/1-hello-world, but see what happens when we try to generate a polymorphic route:

link_to(post.title, post)
# => <a href="/posts/1">hello, world</a>

This is not what we want. Of course, we could set :year and :month parameters manually, but that’s not very DRY. We could also try to override the post_url and post_path methods, but this does not cover all use cases.

Hypothetical solution

Ideally, we would like to return a hash of parameters for the to_param method:

class Post
  def to_param
    { year: created_at.year,
      month: created_at.month,
      id: id }
  end
end

Rails would then have to interpolate these params into the generated URL. Alas, this doesn’t work.

Actual, hacky solution

We can hack around it, tough, by overriding url_for:

class ApplicationController
  def url_for(options = {})
    obj = options[:_positional_args][0]
    if obj === Post
      options[:_positional_keys] = [
        obj.year,
        obj.month,
        obj
      ]
    end
    super(options)
  end
  helper_method :url_for
end

This works fine for the most part – despite being a terrible hack. The helper_method :url_for line does, however, override ActionView’s normal behaviour of generating only paths instead of full URLs. To get around this, we can create a new helper method that uses the controller method:

module ApplicationHelper
  def url_for(options = {})
    return super unless options.is_a? Hash
    controller.url_for(
      options.reverse_merge(only_path: true)
    )
  end
end

Now all URLs are properly generated:

link_to post.title, post
# => <a href="/2011/2/1-hello,world">Hello, world</a>

A note of warning

The code in these examples is extremely simplified. Being a hack, you need to add some proper checks for argument types and the contents of the :_positional_args and :_positional_keys options. Also, you can probably not depend on this behaviour remaining on subsequent versions of Rails. But it still just might come in useful some time…