config.ru

#########################################
# Very basic rack application showing how to use a router based on the uri
# and how to process requests based on the HTTP method used.
#
# Usage:
# $ rackup config.ru
#
# $ curl -X POST -d 'title=retire&body=I should retire in a nice island' localhost:9292/ideas
# $ curl -X POST -d 'title=ask Satish for a gift&body=Find a way to get Satish to send me a gift' localhost:9292/ideas
# $ curl localhost:9292/ideas
#
# More info: https://github.com/rack/rack/wiki/Rack-app-with-uri-and-HTTP-specific-responses
#########################################

require './idea'
run IdeaAPI.new

idea.rb

class Idea
  attr_accessor :title, :body, :created_at

  # Memory store, gets cleared as the process is restarted
  def self.store; @ideas ||= [];  end

  class InvalidParams < StandardError; end
  
  # create an instance based on some passed params
  def initialize(params)
    raise InvalidParams, 'You need to provide at least a title' unless params['title']
    self.title, self.body, self.created_at = params['title'], params['body'], Time.now
  end

  # Converts an instance into a string
  def to_s; "#{title} at #{created_at.to_s}\n#{body}";  end
end

class IdeaAPI
  def call(env)
    case env['REQUEST_METHOD']
    when 'POST'
      Idea.store << Idea.new(request.params)
      [200, {'Content-Type' => 'text/plain'}, ["Idea added, currently #{ Idea.store.size } ideas are in memory!"]]
    when 'GET'
      [200, {'Content-Type' => 'text/plain'}, [Idea.store.map { |idea, idx| idea.to_s }.join("\n\n") + "\n"]]
    end if env['PATH_INFO'] == '/ideas'
    [404, {}, ['Did you get lost?']]
  rescue Idea::InvalidParams => error
    [400, {'Content-Type' => 'text/plain'}, [error.message]]
  end
end