Why does Rails generate a config.ru that runs a class, not an object?

Question

Rack specifies

A Rack application is a Ruby object (not a class) that responds to call.

Consequently, a simple config.ru looks like this:

class MyApp
  def call(env)
    [200, {"Content-Type" => "text/plain"}, ["Hello from Rack!\n"]]
  end
end

run MyApp.new

while Rails generates this:

# This file is used by Rack-based servers to start the application.

require ::File.expand_path('../config/environment',  __FILE__)
run RailsApp::Application

So, I'm wondering: Why not run RailsApp::Application.new when Rack specifies it to be an object, not a class? Is there something special about Rails::Application that I'm missing?

Answer 1

This is indeed a bit hidden :)

RailsApp::Application is a child class of Rails::Application, which in turn is a Rails::Engine which is a Rails::Railtie. Now Rails::Railtie has an inherited hook which is called whenever a child class inherits from the Railtie class (in this case Engine).

This callback includes the Rails::Railtie::Configurable module into the subclass. In this module, you find the first part of the magic.

The method_missing method which gets defined on the class calls the method on an instance of the class, which more or less resolves to

RailsApp::Application.new.call(...)

This call instance method is defined in Rails::Application#call and does the typical Rack handing.

There is probably still a bit more magic involved which makes it not 100% equivalent but that should roughly be it...

Answer 2

A Class is also an Object. Rack doesn't instantiate the app object (which is why it specifies you need to provide the object), you do that in config.ru, so provided the Rails class object follows all the Rack rules when Rack sends 'call', it should not be a problem.

Internally, I don't know whether Rails does anything special on call. It could even be a factory method that both spits out an instance of the application class and runs it. But it doesn't need to be to satisfy Rack.