How does bundler work?

Question

This question is about bundler,rubygems & how does it figure out which method I am calling. Its quite long, so please bear with me.

As per my understanding, Bundler is a dependency management tool for managing gems for ruby. It install all the gems & their dependencies listed in the Gemfile.

The question I want to ask can be best illustrated by an example. so here--> In my rails app I am doing this in my controller:-

module SurveyorControllerCustomMethods
  def create
    super
  end
end
class SurveyorController < ApplicationController
   include Surveyor::SurveyorControllerMethods
end

Here, I am doing two things:

1. Include SurveyorControllerMethods from Surveyor gem.
2. As I have used super here, it would call create method from SurveyorControllerMethods which works just fine but I do not understand it.

and the gem is installed at

$ bundle show surveyor
/home/gaurish/.rvm/gems/ruby-1.9.3-p194/gems/surveyor-0.22.0

Which surprisingly is NOT present in ruby's $LOAD_PATH. so question is:

1. how does it even work?
2. Install a gem using bundler, what happens behind the scenes during install?
3. there are some gems ex jquery-rails for which we even don't have to include/call them in our code & yet, jQuery JavaScript file is automatically included. I know its not magic, so how does this work?

Answer 1

They key is actually in how Rails is set up out of the box. Which is also one of the reasons Rails is so slow to start on larger projects.[1]

In boot.rb you'll find:

require 'bunder/setup'

and at the top of application.rb you'll find:

Bundler.require(:default, :development)

This last method call looks back to your Gemfile, then iterates all of the gems that are in the given groups (:default and :development in the above). It then takes a stab at determining what the default file is that should be loaded, which is typically the gem name, but can also be specified by a :require option. Bundler has some built-in naming convention exceptions too, such as the handling of "-" instead of "_".

Bundler then just loads the primary file for the gem, which in turn (typically) loads the files inside the gem.

The 'bundler/setup' file performs Bundler.setup, which takes care of the load path. It prepends (unshifts) to the $LOAD_PATH for each gem. The actual information about what those loads paths should be comes from rubygems.

Take a look at the source in:

https://github.com/carlhuda/bundler/blob/master/lib/bundler/setup.rb https://github.com/carlhuda/bundler/blob/master/lib/bundler/runtime.rb

[1]: When you start a new Rails project, you'll probably find it doesn't get so slow over time if you delete the Bundler.require line and just require gems as they are needed.