Autoloading and Reloading Constants (Zeitwerk Mode)

This guide documents how autoloading and reloading works in zeitwerk mode.

After reading this guide, you will know:

• Autoloading modes
• Related Rails configuration
• Project structure
• Autoloading, reloading, and eager loading
• Single Table Inheritance
• And more

Chapters

1. Introduction
2. Enabling Zeitwerk Mode
3. Project Structure
4. Autoload paths
5. $LOAD_PATH
6. Reloading
   • Reloading and Stale Objects
7. Eager Loading
8. Single Table Inheritance
9. Troubleshooting
10. Rails.autoloaders
11. Opting Out

1 Introduction

In a normal Ruby program, dependencies need to be loaded by hand. For example, the following controller uses classes ApplicationController and Post, and normally you'd need to put require calls for them:

# DO NOT DO THIS.
require "application_controller"
require "post"
# DO NOT DO THIS.

class PostsController < ApplicationController
  def index
    @posts = Post.all
  end
end

This is not the case in Rails applications, where application classes and modules are just available everywhere:

class PostsController < ApplicationController
  def index
    @posts = Post.all
  end
end

Idiomatic Rails applications only issue require calls to load stuff from their lib directory, the Ruby standard library, Ruby gems, etc. That is, anything that does not belong to their autoload paths, explained below.

2 Enabling Zeitwerk Mode

The autoloading zeitwerk mode is enabled by default in Rails 6 applications running on CRuby:

# config/application.rb
config.load_defaults "6.0" # enables zeitwerk mode in CRuby

In zeitwerk mode, Rails uses Zeitwerk internally to autoload, reload, and eager load. Rails instantiates and configures a dedicated Zeitwerk instance that manages the project.

3 Project Structure

In a Rails application file names have to match the constants they define, with directories acting as namespaces.

For example, the file app/helpers/users_helper.rb should define UsersHelper and the file app/controllers/admin/payments_controller.rb should define Admin::PaymentsController.

Rails configures Zeitwerk to inflect file names with String#camelize. For example, it expects that app/controllers/users_controller.rb defines the constant UsersController because

"users_controller".camelize # => UsersController

If you need to customize any of these inflections, for example to add an acronym, please have a look at config/initializers/inflections.rb.

Please, check the Zeitwerk documentation for further details.

4 Autoload paths

We call autoload paths to the list of application directories whose contents are to be autoloaded. For example, app/models. Such directories represent the root namespace: Object.

Within an autoload path, file names must match the constants they define as documented here.

By default, the autoload paths of an application consist of all the subdirectories of app that exist when the application boots ---except for assets, javascripts, views,--- plus the autoload paths of engines it might depend on.

For example, if UsersHelper is implemented in app/helpers/users_helper.rb, the module is autoloadable, you do not need (and should not write) a require call for it:

$ bin/rails runner 'p UsersHelper'
UsersHelper

Autoload paths automatically pick any custom directories under app. For example, if your application has app/presenters, or app/services, etc., they are added to autoload paths.

The array of autoload paths can be extended by mutating config.autoload_paths, in config/application.rb, but nowadays this is discouraged.

5 $LOAD_PATH

Autoload paths are added to $LOAD_PATH by default. However, Zeitwerk uses absolute file names internally, and your application should not issue require calls for autoloadable files, so those directories are actually not needed there. You can opt-out with this flag:

config.add_autoload_paths_to_load_path = false

That may speed legit require calls a bit, since there are less lookups. Also, if your application uses Bootsnap, that saves the library from building unnecessary indexes, and saves the RAM they would need.

6 Reloading

Rails automatically reloads classes and modules if application files change.

More precisely, if the web server is running and application files have been modified, Rails unloads all autoloaded constants just before the next request is processed. That way, application classes or modules used during that request are going to be autoloaded, thus picking up their current implementation in the file system.

Reloading can be enabled or disabled. The setting that controls this behavior is config.cache_classes, which is false by default in development mode (reloading enabled), and true by default in production mode (reloading disabled).

Rails detects files have changed using an evented file monitor (default), or walking the autoload paths, depending on config.file_watcher.

In a Rails console there is no file watcher active regardless of the value of config.cache_classes. This is so because, normally, it would be confusing to have code reloaded in the middle of a console session, the same way you generally want an individual request to be served by a consistent, non-changing set of application classes and modules.

However, you can force a reload in the console executing reload!:

$ bin/rails c
Loading development environment (Rails 6.0.0)
irb(main):001:0> User.object_id
=> 70136277390120
irb(main):002:0> reload!
Reloading...
=> true
irb(main):003:0> User.object_id
=> 70136284426020

as you can see, the class object stored in the User constant is different after reloading.

6.1 Reloading and Stale Objects

It is very important to understand that Ruby does not have a way to truly reload classes and modules in memory, and have that reflected everywhere they are already used. Technically, "unloading" the User class means removing the User constant via Object.send(:remove_const, "User").

Therefore, if you store a reloadable class or module object in a place that is not reloaded, that value is going to become stale.

For example, if an initializer stores and caches a certain class object

# config/initializers/configure_payment_gateway.rb
# DO NOT DO THIS.
$PAYMENT_GATEWAY = Rails.env.production? ? RealGateway : MockedGateway
# DO NOT DO THIS.

and MockedGateway gets reloaded, $PAYMENT_GATEWAY still stores the class object MockedGateway evaluated to when the initializer ran. Reloading does not change the class object stored in $PAYMENT_GATEWAY.

Similarly, in the Rails console, if you have a user instance and reload:

> user = User.new
> reload!

the user object is instance of a stale class object. Ruby gives you a new class if you evaluate User again, but does not update the class user is instance of.

Another use case of this gotcha is subclassing reloadable classes in a place that is not reloaded:

# lib/vip_user.rb
class VipUser < User
end

if User is reloaded, since VipUser is not, the superclass of VipUser is the original stale class object.

Bottom line: do not cache reloadable classes or modules.

7 Eager Loading

In production-like environments it is generally better to load all the application code when the application boots. Eager loading puts everything in memory ready to serve requests right away, and it is also CoW-friendly.

Eager loading is controlled by the flag config.eager_load, which is enabled by default in production mode.

The order in which files are eager loaded is undefined.

if the Zeitwerk constant is defined, Rails invokes Zeitwerk::Loader.eager_load_all regardless of the application autoloading mode. That ensures dependencies managed by Zeitwerk are eager loaded.

8 Single Table Inheritance

Single Table Inheritance is a feature that doesn't play well with lazy loading. Reason is, its API generally needs to be able to enumerate the STI hierarchy to work correctly, whereas lazy loading defers loading classes until they are referenced. You can't enumerate what you haven't referenced yet.

In a sense, applications need to eager load STI hierarchies regardless of the loading mode.

Of course, if the application eager loads on boot, that is already accomplished. When it does not, it is in practice enough to instantiate the existing types in the database, which in development or test modes is usually fine. One way to do that is to throw this module into the lib directory:

module StiPreload
  unless Rails.application.config.eager_load
    extend ActiveSupport::Concern

    included do
      cattr_accessor :preloaded, instance_accessor: false
    end

    class_methods do
      def descendants
        preload_sti unless preloaded
        super
      end

      # Constantizes all types present in the database. There might be more on
      # disk, but that does not matter in practice as far as the STI API is
      # concerned.
      #
      # Assumes store_full_sti_class is true, the default.
      def preload_sti
        types_in_db = \
          base_class.
            select(inheritance_column).
            distinct.
            pluck(inheritance_column).
            compact.
            each(&:constantize)

        types_in_db.each do |type|
          logger.debug("Preloading STI type #{type}")
          type.constantize
        end

        self.preloaded = true
      end
    end
  end
end

and then include it in the STI root classes of your project:

# app/models/shape.rb
require "sti_preload"

class Shape < ApplicationRecord
  include StiPreload # Only in the root class.
end

# app/models/polygon.rb
class Polygon < Shape
end

# app/models/triangle.rb
class Triangle < Polygon
end

9 Troubleshooting

The best way to follow what the loaders are doing is to inspect their activity.

The easiest way to do that is to throw

Rails.autoloaders.log!

to config/application.rb after loading the framework defaults. That will print traces to standard output.

If you prefer logging to a file, configure this instead:

Rails.autoloaders.logger = Logger.new("#{Rails.root}/log/autoloading.log")

The Rails logger is still not ready in config/application.rb, but it is in initializers:

# config/initializers/log_autoloaders.rb
Rails.autoloaders.logger = Rails.logger

10 Rails.autoloaders

The Zeitwerk instances managing your application are available at

Rails.autoloaders.main
Rails.autoloaders.once

The former is the main one. The latter is there mostly for backwards compatibility reasons, in case the application has something in config.autoload_once_paths (this is discouraged nowadays).

You can check if zeitwerk mode is enabled with

Rails.autoloaders.zeitwerk_enabled?

11 Opting Out

Applications can load Rails 6 defaults and still use the classic autoloader this way:

# config/application.rb
config.load_defaults "6.0"
config.autoloader = :classic

That may be handy if upgrading to Rails 6 in different phases, but classic mode is discouraged for new applications.

zeitwerk mode is not available in versions of Rails previous to 6.0.

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