TinyRails

I've written Rails for years. I wanted to know what it does under the hood, so I worked through Noah Gibbs' Rebuilding Rails and built my own small version. It's one of the projects I'm proudest of.

TinyRails does the core of what a web framework does:

• Rack integration, so it runs on any Ruby web server
• Routing by convention: /tweets/show goes to TweetsController#show
• Controllers that render ERB views from app/views/{controller}/{action}.html.erb
• A model layer that stores records as JSON files, with find and all
• Helpful errors: a typo in a controller name tells you what it looked for

To make sure it worked for real, I built a small app on top of it: best_tweets.

The whole request, in one method

This is the entry point, trimmed a little. Rack calls it with the request, it finds a controller and an action, runs it, and hands back status, headers and body:

module Tinyrails
  class Application
    def call(env)
      return [404, { "content-type" => "text/html" }, []] if env["PATH_INFO"] == "/favicon.ico"

      klass, id, act = get_controller_and_action(env)
      env["route.id"] = id if id

      controller = klass.new(env)
      controller.send(act)
      response = controller.get_response

      unless response
        controller.render(act)
        response = controller.get_response
      end

      [response.status, response.headers, response.body]
    rescue NameError => e
      no_controller_error(e)
    rescue NoMethodError => e
      no_action_error(e)
    end
  end
end

Finding classes by name

Rails finds TweetsController without a require. TinyRails does it the way the book does: when Ruby can't find a constant, it turns the name into a file name and tries to load it.

class Object
  def self.const_missing(c)
    begin
      require Tinyrails.to_underscore(c.to_s)
    rescue LoadError => e
      raise NameError, "#{c} was not found. Did you spell it correctly? Additional info: #{e}"
    end

    raise NameError, "Found #{Tinyrails.to_underscore(c.to_s)}.rb, but #{c} was not defined" unless const_defined?(c)

    const_get(c)
  end
end

Modern Rails uses Zeitwerk for this instead, which maps file names to constants up front.