Autoloading and Reloading Constants

1. Introduction

In an ordinary Ruby program, you explicitly load the files that define classes and modules you want to use. You normally issue require calls. This is not the case in Rails applications, where application classes and modules are just available everywhere without require calls. This is possible thanks to a couple of Zeitwerk loaders Rails sets up on your behalf, which provide autoloading, reloading, and eager loading.

That convenience is easy to over-read. The next sentence from the Rails Guides is the real boundary:

On the other hand, those loads do not manage anything else. In particular, they do not manage the Ruby standard library, gem dependencies, Rails components, or even (by default) the application lib directory. That code has to be loaded as usual.

Read it as a hard scope cut, not a footnote:

Zeitwerk does manage	Zeitwerk does not manage
App code under app/ (models, controllers, jobs, …)	Ruby standard library (Set, JSON, …)
Autoload / reload / eager load for those paths	Gem dependencies (nokogiri, sidekiq, …)
Constant ↔ file mapping Rails configured for you	Rails framework components themselves
	Application lib/ by default

So: User, OrdersController, ChargeJob can appear without require. But require "set", Bundler-loaded gems, Rails internals, and (unless you opt in) anything under lib/ still follow ordinary Ruby loading. Autoloading is not a global replacement for require—it is a managed island for application constants.