learning RoR, can someone explain this boot.rb from H**ll? - ruby-on-rails

Question

I am looking at this boot.rb file:
http://github.com/bestbuyremix/BBYIDX/blob/master/config/boot.rb
And after trying to understand it, it is as if I have learned nothing so far.
Can someone detail what is going on here?
I have no idea how someone could even come up with this?
# Don't change this file!
# Configure your app in config/environment.rb and config/environments/*.rb
RAILS_ROOT = "#{File.dirname(__FILE__)}/.." unless defined?(RAILS_ROOT)
module Rails
class << self
def boot!
unless booted?
preinitialize
pick_boot.run
end
end
def booted?
defined? Rails::Initializer
end
def pick_boot
(vendor_rails? ? VendorBoot : GemBoot).new
end
def vendor_rails?
File.exist?("#{RAILS_ROOT}/vendor/rails")
end
def preinitialize
load(preinitializer_path) if File.exist?(preinitializer_path)
end
def preinitializer_path
"#{RAILS_ROOT}/config/preinitializer.rb"
end
end
class Boot
def run
load_initializer
Rails::Initializer.run(:set_load_path)
end
end
class VendorBoot < Boot
def load_initializer
require "#{RAILS_ROOT}/vendor/rails/railties/lib/initializer"
Rails::Initializer.run(:install_gem_spec_stubs)
end
end
class GemBoot < Boot
def load_initializer
self.class.load_rubygems
load_rails_gem
require 'initializer'
end
def load_rails_gem
if version = self.class.gem_version
gem 'rails', version
else
gem 'rails'
end
rescue Gem::LoadError => load_error
$stderr.puts %(Missing the Rails #{version} gem. Please `gem install -v=#{version} rails`, update your RAILS_GEM_VERSION setting in config/environment.rb for the Rails version you do have installed, or comment out RAILS_GEM_VERSION to use the latest version installed.)
exit 1
end
class << self
def rubygems_version
Gem::RubyGemsVersion if defined? Gem::RubyGemsVersion
end
def gem_version
if defined? RAILS_GEM_VERSION
RAILS_GEM_VERSION
elsif ENV.include?('RAILS_GEM_VERSION')
ENV['RAILS_GEM_VERSION']
else
parse_gem_version(read_environment_rb)
end
end
def load_rubygems
require 'rubygems'
min_version = '1.1.1'
unless rubygems_version >= min_version
$stderr.puts %Q(Rails requires RubyGems >= #{min_version} (you have #{rubygems_version}). Please `gem update --system` and try again.)
exit 1
end
rescue LoadError
$stderr.puts %Q(Rails requires RubyGems >= #{min_version}. Please install RubyGems and try again: http://rubygems.rubyforge.org)
exit 1
end
def parse_gem_version(text)
$1 if text =~ /^[^#]*RAILS_GEM_VERSION\s*=\s*["']([!~<>=]*\s*[\d.]+)["']/
end
private
def read_environment_rb
File.read("#{RAILS_ROOT}/config/environment.rb")
end
end
end
end
# All that for this:
Rails.boot!

Answer 1

If you are learning Rails, this is not the place to do it. Perhaps you have the thought that in order to use it, you need to understand how the code flows from the beginning? Don't do that. :)
If you're learning rails, use any of the many guides and tutorials to build a basic site.
As for this bit of code, some concepts that it employs involve ruby's iconic class << self Here is a critical read on meta classes: http://yehudakatz.com/2009/11/15/metaprogramming-in-ruby-its-all-about-the-self/
The Rails.boot! at the bottom leads you to the conclusion "the method boot! is called on the object Rails ... going back up to the top, you see
module Rails
class << self
def boot!
unless booted?
preinitialize
pick_boot.run
end
end
...
Here you can see the magic behind class << self ... it created the boot! method on the module itself. from there you can trace the method call throughout the file, as it checks for the existence of a preinitializer file...
pick_boot returns an object, either VendorBoot or GemBoot depending on the result of vendor_rails? and then call the run method on it.
From there you have some standard class inheritance of the Boot classes, as it sets up the rest of the libraries. Hopefully that gets you started. :)
This is actually very good OO style... small methods and classes that all do a simple task. There's also OO inheritance and several common ruby idioms. All in all, a very good bit of ruby code. :)
Update
Here's a rough estimate of how it would look if coded in a more procedural style:
RAILS_ROOT = "#{File.dirname(__FILE__)}/.." unless defined?(RAILS_ROOT)
unless defined? Rails::Initializer
preinitializer_path = "#{RAILS_ROOT}/config/preinitializer.rb"
load() if File.exist?(preinitializer_path)
if File.exist?("#{RAILS_ROOT}/vendor/rails")
require "#{RAILS_ROOT}/vendor/rails/railties/lib/initializer"
Rails::Initializer.run(:install_gem_spec_stubs)
Rails::Initializer.run(:set_load_path)
else
begin
require 'rubygems'
min_version = '1.1.1'
rubygems_version = Gem::RubyGemsVersion if defined? Gem::RubyGemsVersion
unless rubygems_version >= min_version
$stderr.puts %Q(Rails requires RubyGems >= #{min_version} (you have #{rubygems_version}). Please `gem update --system` and try again.)
exit 1
end
rescue LoadError
$stderr.puts %Q(Rails requires RubyGems >= #{min_version}. Please install RubyGems and try again: http://rubygems.rubyforge.org)
exit 1
end
begin
if defined? RAILS_GEM_VERSION
version = RAILS_GEM_VERSION
elsif ENV.include?('RAILS_GEM_VERSION')
version = ENV['RAILS_GEM_VERSION']
else
version = $1 if (File.read("#{RAILS_ROOT}/config/environment.rb")) =~ /^[^#]*RAILS_GEM_VERSION\s*=\s*["']([!~=]*\s*[\d.]+)["']/
end
if version
gem 'rails', version
else
gem 'rails'
end
rescue Gem::LoadError => load_error
$stderr.puts %(Missing the Rails #{version} gem. Please `gem install -v=#{version} rails`, update your RAILS_GEM_VERSION setting in config/environment.rb for the Rails version you do have installed, or comment out RAILS_GEM_VERSION to use the latest version installed.)
exit 1
end
require 'initializer'
end
end

Answer 2

What this is really doing is a bunch of fancy footwork to determine where the rails source code is.
You have the ability to 'freeze' your current rails gem source code into the vendor directory. This is useful when you might have more than one version of rails installed and want to make sure that your application is developed and run with that version only.
This code is checking to see if a version of rails has been frozen into the vendor directory, and if so, use that. If a frozen version isn't available it tries to use the local gems, but first makes sure they meet a minimum version requirement.
For more information on 'freezing' your gems, look at the descriptions of the rake tasks availble to your project with "rake -T".