Why does Rails generate config.ru, which runs the class, and not the object?

Question

Rack indicates

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

Therefore, 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 indicates that it is an object, not a class? Is there something special about Rails::Application that I am missing?

Answer 1

It really is a little hidden :)

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

This callback includes the Rails::Railtie::Configurable module in a subclass. In this module you will find the first part of magic.

The method_missing method, which is defined in the class, calls the method in the class instance, which more or less allows

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

This call instance method is defined in Rails::Application#call and performs a typical rack transfer.

There is probably still a bit more magic, which makes it not the equivalent of 100%, but it should be something like this ...

Answer 2

The class is also an object. Rack does not instantiate the application object (which is why it indicates that you need to provide the object), you do it in config.ru , therefore, if the Rails class class follows all Rack rules, when Rack sends a “call”, it should not be a problem.

Inside, I don't know if Rails does anything special on call . It could even be a factory method that spits out an instance of an application class and runs it. But it is not necessary to satisfy Rack.