SmartUrlHelper - configurable replacement for url_for. See http://lostechies.com/joshuaflanagan/2012/03/27/a-smarter-rails-url_for-helper/

example_config_initializer_smart_urls.rb

SmartUrlHelper.configure do |url|
  url.for ->model{ model.is_a?(Comment) } do |helpers, model|
    helpers.post_comment_path(model.post, model)
  end

  # TODO: Add a convenience method for matching on type:
  # url.for(Comment) do |helpers, model|
  #   helpers.post_comment_path(model.post, model)
  # end
end

smart_url_helper.rb

module SmartUrlHelper
  # A smarter replacement for #url_for. First checks to see if a
  # special-case handler has been registered for the given model. If so,
  # use that to generate the URL. If not, fall back to Rails' #url_for helper.
  # Special-case handlers should be registered in a config/initializer that
  # calls SmartUrlHelper.configure.
  #
  # Returns the default URL for the given model.
  def smart_url_for(model, *args)
    url = ::SmartUrlHelper::SmartUrlConfiguration.lookup_smart_url(self, model, *args)
    return url if url.present?
    url_for(model, *args)
  end

  # Call in config/initializer to register model handlers.
  # Takes a block for an instance of SmartUlrConfiguration.
  def self.configure
    Rails.application.config.smart_url_handlers = []
    yield ::SmartUrlHelper::SmartUrlConfiguration.new
  end

  # Provides the DSL for registering model handlers in SmartUrlHelper.configure
  class SmartUrlConfiguration
    # Registers a handler to generate a URL for a model that matches the predicate
    #
    #  predicate     - A lambda that takes in a model instance and returns true
    #                  if the handler can generate a URL for the given model.
    #  url_generator - A block that builds a url for the given model.
    #                  The block will receive a helper object and the model
    #                  as parameters.
    def for(predicate, &url_generator)
      self.class.handlers << [predicate, url_generator]
    end

    def self.handlers
      Rails.application.config.smart_url_handlers
    end

    def self.lookup_smart_url(context, model, *args)
      handler = self.handlers.detect{|h| h[0].call(model)}
      return nil unless handler.present?
      handler[1].call(context, model)
    end
  end
end