Rails 3 Initialization Order

Question

Can someone please point me in the right direction for the order in which rails modules get instantiated.

The main things I'm trying to find are:

1) When do gems get loaded?

2) When do config/initializers/* get loaded?

3) When do named routes in routes.rb get processed?

Answer 1

Answering your question is easy by adding some puts statements in your Rails application. (It seems like a lot of people are reluctant to dig in and do this, but I really recommend it!) So, by experimentation and observation alone, here is the order of the things you mentioned:

1. boot.rb
2. config/initializers/*
3. routes.rb

Here is a little more detail:

1. boot.rb

This loads the application gems by using bundler:

require 'rubygems'
# Set up gems listed in the Gemfile.
ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile', __FILE__)
require 'bundler/setup' if File.exists?(ENV['BUNDLE_GEMFILE'])

2. config/initializers/*

They run in alphabetical order.

If you are curious what triggers this, take a look at engine.rb in the railties source code. (It is useful to know that a Rails Application is a subclass of a Rails Engine.)

initializer :load_config_initializers do
  config.paths["config/initializers"].existent.sort.each do |initializer|
    load(initializer)
  end
end

3. routes.rb

By observation, I see that route drawing (specification) occurs next.

But looking at the details is more involved, so if you are interested I would read SO: Controlling routes loading order from Engines and perhaps take a look at the :add_routing_paths initializer in engine.rb.

Answer 2

Check out this insanely detailed (and long) piece of documentation on the initialization process:

http://guides.rubyonrails.org/initialization.html

Answer 3

I started a console in Rails 3 and here is the order:

• script/rails
• config/boot.rb
• config/application.rb
• config/environment.rb
• config/initializers/*.rb (In alphabetic order)