post.rb

class Post
  def self.find_by_id(id)
    self.new
  end

  def comments
    CommentCollection.new
  end
end

class Comment
  attr_reader :title, :body
  def initialize
    @title = "Anonymous Comment"
    @body = "Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nam non mauris at neque mollis pellentesque"
  end
end

class CommentCollection
  def find_by_id(id)
    Comment.new
  end
end

with-renee.ru

require './post'
require 'renee'

run Renee {
  path('posts') do
    var :int do |post_id|
      post = Post.find_by_id(post_id)
      path('comments') do
        var :int do |comment_id|
          comment = post.comments.find_by_id(comment_id)
          get { 
            halt "#{comment.title} - #{comment.body}" 
          }
        end
      end
    end
  end
}

with-sinatra.ru

require './post'
require 'sinatra/base'

class MyApp < Sinatra::Base
  get "/posts/:post_id/comments/:comment_id" do
    post = Post.find_by_id(params[:post_id])
    comment = post.comments.find_by_id(params[:comment_id])
    "#{comment.title} - #{comment.body}"
  end
end

run MyApp