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Rebuild Rails Part 3

Let’s continue our rebuild rails series by supporting controllers. We simplify things by assuming a controller/action path format and only support GET request.

We adjust our .call implementation to handle the new path format. We also introduce another method #render_controller_action that inspects the path_info and instantiates the right controller using Rails NameController convention.

# lib/tracks.rb
def self.call(env)
  path_info = env["PATH_INFO"]
  if  path_info == "/"
    text = Tracks::Controller.render_default_root
  else
    text = Tracks::Controller.render_controller_action(env)
  end

  [200, {"Content-Type" => "text/html"}, [text] ]
end

# lib/tracks/controller.rb
def self.render_controller_action(env)
  path_info = env["PATH_INFO"]
  _, controller, action, after = path_info.split("/")

  controller_class_name = controller.capitalize + "Controller"
  controller_class = Object.const_get(controller_class_name)
  controller_class.new.send(action)
end

$ ruby spec/application_spec.rb
Run options: --seed 55835
# Running:

.E

Finished in 0.015837s, 126.2865 runs/s, 126.2865 assertions/s.

  1) Error:
CrazyApp::Application#test_0002_should respond with different path:
NameError: uninitialized constant PostsController
    /code/crazy/lib/tracks/controller.rb:13:in `const_get'

When we run the test, you see it failed on PostsController which is what we expect since we haven’t implemented PostsController yet. Let’s add the controller now.

# app/controllers/posts_controller.rb
class PostsController < Tracks::Controller
  def index
    "hello from tracks /posts/index"
  end
end

# config/application.rb
require "./app/controllers/posts_controller"

$ ruby spec/application_spec.rb
Run options: --seed 2475

# Running:
..

Finished in 0.019152s, 104.4277 runs/s, 208.8555 assertions/s.

2 runs, 4 assertions, 0 failures, 0 errors, 0 skips

Automatic loading

Our tests pass but something’s not right. In Rails, there is no need to require every controller (or pretty much anything) to make it work. To support this feature in our framework, we need 2 things:

  • convert PostsController to posts_controller.rb; and
  • auto-require ‘posts_controller’

To tie these 2 together, we tap into Object.const_missing so our framework would know if a class has been used but not yet loaded.

We also update the $LOAD_PATH to include app/controllers folder so Ruby knows where to look.

# lib/tracks.rb
require File.expand_path("../tracks/helper", __FILE__)
require File.expand_path("../tracks/object", __FILE__)

# config/application.rb
require './lib/tracks'
$LOAD_PATH << File.expand_path("../../app/controllers", __FILE__)

module CrazyApp
  class Application < Tracks::Application
  end
end

# lib/tracks/helper.rb
module Tracks
  module Helper
    def self.to_underscore(string)
      string.scan(/[A-Z][a-z]+/).
      join('_').
      downcase
    end
  end
end

# lib/tracks/object.rb
class Object
  def self.const_missing(c)
    require Tracks::Helper.to_underscore(c.to_s)
    const_get(c)
  end
end

Our implementation of .to_underscore is limited compared to what’s supported in Rails.

irb(main):019:0> s = "PostsController"
=> "PostsController"
irb(main):020:0> s.scan(/[A-Z][a-z]+/).join('_').downcase
=> "posts_controller"

Also, since we call const_get inside const_missing, you will run into serious trouble if your file does not contain the expected class. Try changing the name of the class inside app/controllers/posts_controller.rb into something else and you will get this error.

$ ruby spec/application_spec.rb
Run options: --seed 60433

# Running:

.E
Finished in 0.043152s, 46.3478 runs/s, 46.3478 assertions/s.

  1) Error:
CrazyApp::Application#test_0002_should respond with different path:
SystemStackError: stack level too deep
    /Users/greg/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/forwardable.rb:174

2 runs, 2 assertions, 0 failures, 1 errors, 0 skips
20 Nov 2014