How does ruby gem works in a rails environment

Question

I am trying to understand in rails, how ruby gems become available to used automatically without being required in the files that are using the gems?

Answer 1

This is done through bundler/setup: http://bundler.io/v1.3/bundler_setup.html. It is required inside your config/boot.rb file. In short it firstly sets environmental variable to point to your Gemfile:

ENV['BUNDLE_GEMFILE'] ||= File.expand_path('../../Gemfile', __FILE__)

Then it adds paths for all your gems to LOAD_PATH, by requiring bundler/setup:

require 'bundler/setup' if File.exist?(ENV['BUNDLE_GEMFILE'])

Then it requires all the needed gems (config/application.rb):

Bundler.require(*Rails.groups)

Answer 2

Rails applications use bundler (that's the thing using the Gemfile). When bundler loads the Gemfile on startup of a rails application, it automatically requires all gems listed there, thus you don't have to do this yourself.

Answer 3

I recomend you to read "Crafting Rails 4 Applications: Expert Practices for Everyday Rails Development" Chapter 1. Creating Our Own Renderer:

Notice the gem has the same name as the file inside the lib directory, which is pdf_renderer . By following this convention, whenever you declare this gem in a Rails application’s Gemfile , the file at lib/pdf_renderer.rb will be automatically required.