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. For example, the following controller refers to ApplicationController and Post, and you'd normally issue 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 without require calls:

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

Rails autoloads them on your behalf if needed. This is possible thanks to a couple of Zeitwerk loaders Rails sets up on your behalf, which provide autoloading, reloading, and eager loading.

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

2. 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.

By default, 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 that is what "users_controller".camelize returns.

The section Customizing Inflections below documents ways to override this default.

Please, check the Zeitwerk documentation for further details.

3. config.autoload_paths

We refer to the list of application directories whose contents are to be autoloaded and (optionally) reloaded as autoload paths. 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, javascript, and 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

Rails adds custom directories under app to the autoload paths automatically. For example, if your application has app/presenters, you don't need to configure anything in order to autoload presenters; it works out of the box.

The array of default autoload paths can be extended by pushing to config.autoload_paths, in config/application.rb or config/environments/*.rb. For example:

module MyApplication
  class Application < Rails::Application
    config.autoload_paths << "#{root}/extras"
  end
end

Also, engines can push in body of the engine class and in their own config/environments/*.rb.

The autoload paths are managed by the Rails.autoloaders.main autoloader.

4. config.autoload_lib(ignore:)

By default, the lib directory does not belong to the autoload paths of applications or engines.

The configuration method config.autoload_lib adds the lib directory to config.autoload_paths and config.eager_load_paths. It has to be invoked from config/application.rb or config/environments/*.rb, and it is not available for engines.

Normally, lib has subdirectories that should not be managed by the autoloaders. Please, pass their name relative to lib in the required ignore keyword argument. For example:

config.autoload_lib(ignore: %w(assets tasks))

Why? While assets and tasks share the lib directory with regular Ruby code, their contents are not meant to be reloaded or eager loaded.

The ignore list should have all lib subdirectories that do not contain files with .rb extension, or that should not be reloaded or eager loaded. For example,

config.autoload_lib(ignore: %w(assets tasks templates generators middleware))

config.autoload_lib is not available before 7.1, but you can still emulate it as long as the application uses Zeitwerk:

# config/application.rb
module MyApp
  class Application < Rails::Application
    lib = root.join("lib")

    config.autoload_paths << lib
    config.eager_load_paths << lib

    Rails.autoloaders.main.ignore(
      lib.join("assets"),
      lib.join("tasks"),
      lib.join("generators")
    )

    # ...
  end
end

5. config.autoload_once_paths

You may want to be able to autoload classes and modules without reloading them. The autoload_once_paths configuration stores code that can be autoloaded, but won't be reloaded.

By default, this collection is empty, but you can extend it pushing to config.autoload_once_paths. You can do so in config/application.rb or config/environments/*.rb. For example:

module MyApplication
  class Application < Rails::Application
    config.autoload_once_paths << "#{root}/app/serializers"
  end
end

Also, engines can push in body of the engine class and in their own config/environments/*.rb.

This is key for classes and modules that are cached in places that survive reloads, like the Rails framework itself.

For example, Active Job serializers are stored inside Active Job:

# config/initializers/custom_serializers.rb
Rails.application.config.active_job.custom_serializers << MoneySerializer

and Active Job itself is not reloaded when there's a reload, only application and engines code in the autoload paths is.

Making MoneySerializer reloadable would be confusing, because reloading an edited version would have no effect on that class object stored in Active Job. Indeed, if MoneySerializer was reloadable, starting with Rails 7 such initializer would raise a NameError.

Another use case is when engines decorate framework classes:

initializer "decorate ActionController::Base" do
  ActiveSupport.on_load(:action_controller_base) do
    include MyDecoration
  end
end

There, the module object stored in MyDecoration by the time the initializer runs becomes an ancestor of ActionController::Base, and reloading MyDecoration is pointless, it won't affect that ancestor chain.

Classes and modules from the autoload once paths can be autoloaded in config/initializers. So, with that configuration this works:

# config/initializers/custom_serializers.rb
Rails.application.config.active_job.custom_serializers << MoneySerializer

The autoload once paths are managed by Rails.autoloaders.once.

6. config.autoload_lib_once(ignore:)

The method config.autoload_lib_once is similar to config.autoload_lib, except that it adds lib to config.autoload_once_paths instead. It has to be invoked from config/application.rb or config/environments/*.rb, and it is not available for engines.

By calling config.autoload_lib_once, classes and modules in lib can be autoloaded, even from application initializers, but won't be reloaded.

config.autoload_lib_once is not available before 7.1, but you can still emulate it as long as the application uses Zeitwerk:

# config/application.rb
module MyApp
  class Application < Rails::Application
    lib = root.join("lib")

    config.autoload_once_paths << lib
    config.eager_load_paths << lib

    Rails.autoloaders.once.ignore(
      lib.join("assets"),
      lib.join("tasks"),
      lib.join("generators")
    )

    # ...
  end
end

7. Reloading

Rails automatically reloads classes and modules if application files in the autoload paths change.

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

Reloading can be enabled or disabled. The setting that controls this behavior is config.enable_reloading, which is true by default in development mode, and false by default in production mode. For backwards compatibility, Rails also supports config.cache_classes, which is equivalent to !config.enable_reloading.

Rails uses an evented file monitor to detect files changes by default. It can be configured instead to detect file changes by walking the autoload paths. This is controlled by the config.file_watcher setting.

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

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

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.

7.1. 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").

For example, check out this Rails console session:

irb> joe = User.new
irb> reload!
irb> alice = User.new
irb> joe.class == alice.class
=> false

joe is an instance of the original User class. When there is a reload, the User constant then evaluates to a different, reloaded class. alice is an instance of the newly loaded User, but joe is not — his class is stale. You may define joe again, start an IRB subsession, or just launch a new console instead of calling reload!.

Another situation in which you may find 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.

8. Autoloading When the Application Boots

While booting, applications can autoload from the autoload once paths, which are managed by the once autoloader. Please check the section config.autoload_once_paths above.

However, you cannot autoload from the autoload paths, which are managed by the main autoloader. This applies to code in config/initializers as well as application or engines initializers.

Why? Initializers only run once, when the application boots. They do not run again on reloads. If an initializer used a reloadable class or module, edits to them would not be reflected in that initial code, thus becoming stale. Therefore, referring to reloadable constants during initialization is disallowed.

Let's see what to do instead.

8.1. Use Case 1: During Boot, Load Reloadable Code

8.1.1. Autoload on Boot and on Each Reload

Let's imagine ApiGateway is a reloadable class and you need to configure its endpoint while the application boots:

# config/initializers/api_gateway_setup.rb
ApiGateway.endpoint = "https://example.com" # NameError

Initializers cannot refer to reloadable constants, you need to wrap that in a to_prepare block, which runs on boot, and after each reload:

# config/initializers/api_gateway_setup.rb
Rails.application.config.to_prepare do
  ApiGateway.endpoint = "https://example.com" # CORRECT
end

8.1.2. Autoload on Boot Only

Reloadable classes and modules can be autoloaded in after_initialize blocks too. These run on boot, but do not run again on reload. In some exceptional cases this may be what you want.

Preflight checks are a use case for this:

# config/initializers/check_admin_presence.rb
Rails.application.config.after_initialize do
  unless Role.where(name: "admin").exists?
    abort "The admin role is not present, please seed the database."
  end
end

8.2. Use Case 2: During Boot, Load Code that Remains Cached

Some configurations take a class or module object, and they store it in a place that is not reloaded. It is important that these are not reloadable, because edits would not be reflected in those cached stale objects.

One example is middleware:

config.middleware.use MyApp::Middleware::Foo

When you reload, the middleware stack is not affected, so it would be confusing that MyApp::Middleware::Foo is reloadable. Changes in its implementation would have no effect.

Another example is Active Job serializers:

# config/initializers/custom_serializers.rb
Rails.application.config.active_job.custom_serializers << MoneySerializer

Whatever MoneySerializer evaluates to during initialization gets pushed to the custom serializers, and that object stays there on reloads.

Yet another example are railties or engines decorating framework classes by including modules. For instance, turbo-rails decorates ActiveRecord::Base this way:

initializer "turbo.broadcastable" do
  ActiveSupport.on_load(:active_record) do
    include Turbo::Broadcastable
  end
end

That adds a module object to the ancestor chain of ActiveRecord::Base. Changes in Turbo::Broadcastable would have no effect if reloaded, the ancestor chain would still have the original one.

Corollary: Those classes or modules cannot be reloadable.

An idiomatic way to organize these files is to put them in the lib directory and load them with require where needed. For example, if the application has custom middleware in lib/middleware, issue a regular require call before configuring it:

require "middleware/my_middleware"
config.middleware.use MyMiddleware

Additionally, if lib is in the autoload paths, configure the autoloader to ignore that subdirectory:

# config/application.rb
config.autoload_lib(ignore: %w(assets tasks ... middleware))

since you are loading those files yourself.

As noted above, another option is to have the directory that defines them in the autoload once paths and autoload. Please check the section about config.autoload_once_paths for details.

8.3. Use Case 3: Configure Application Classes for Engines

Let's suppose an engine works with the reloadable application class that models users, and has a configuration point for it:

# config/initializers/my_engine.rb
MyEngine.configure do |config|
  config.user_model = User # NameError
end

In order to play well with reloadable application code, the engine instead needs applications to configure the name of that class:

# config/initializers/my_engine.rb
MyEngine.configure do |config|
  config.user_model = "User" # OK
end

Then, at run time, config.user_model.constantize gives you the current class object.

9. 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 disabled by default in all environments except production. When a Rake task gets executed, config.eager_load is overridden by config.rake_eager_load, which is false by default. So, by default, in production environments Rake tasks do not eager load the application.

The order in which files are eager-loaded is undefined.

During eager loading, Rails invokes Zeitwerk::Loader.eager_load_all. That ensures all gem dependencies managed by Zeitwerk are eager-loaded too.

10. Single Table Inheritance

Single Table Inheritance doesn't play well with lazy loading: Active Record has to be aware of STI hierarchies to work correctly, but when lazy loading, classes are precisely loaded only on demand!

To address this fundamental mismatch we need to preload STIs. There are a few options to accomplish this, with different trade-offs. Let's see them.

10.1. Option 1: Enable Eager Loading

The easiest way to preload STIs is to enable eager loading by setting:

config.eager_load = true

in config/environments/development.rb and config/environments/test.rb.

This is simple, but may be costly because it eager loads the entire application on boot and on every reload. The trade-off may be worthwhile for small applications, though.

10.2. Option 2: Preload a Collapsed Directory

Store the files that define the hierarchy in a dedicated directory, which makes sense also conceptually. The directory is not meant to represent a namespace, its sole purpose is to group the STI:

app/models/shapes/shape.rb
app/models/shapes/circle.rb
app/models/shapes/square.rb
app/models/shapes/triangle.rb

In this example, we still want app/models/shapes/circle.rb to define Circle, not Shapes::Circle. This may be your personal preference to keep things simple, and also avoids refactors in existing code bases. The collapsing feature of Zeitwerk allows us to do that:

# config/initializers/preload_stis.rb

shapes = "#{Rails.root}/app/models/shapes"
Rails.autoloaders.main.collapse(shapes) # Not a namespace.

unless Rails.application.config.eager_load
  Rails.application.config.to_prepare do
    Rails.autoloaders.main.eager_load_dir(shapes)
  end
end

In this option, we eager load these few files on boot and reload even if the STI is not used. However, unless your application has a lot of STIs, this won't have any measurable impact.

10.3. Option 3: Preload a Regular Directory

Similar to the previous one, but the directory is meant to be a namespace. That is, app/models/shapes/circle.rb is expected to define Shapes::Circle.

For this one, the initializer is the same except no collapsing is configured:

# config/initializers/preload_stis.rb

unless Rails.application.config.eager_load
  Rails.application.config.to_prepare do
    Rails.autoloaders.main.eager_load_dir("#{Rails.root}/app/models/shapes")
  end
end

Same trade-offs.

10.4. Option 4: Preload Types from the Database

In this option we do not need to organize the files in any way, but we hit the database:

# config/initializers/preload_stis.rb

unless Rails.application.config.eager_load
  Rails.application.config.to_prepare do
    types = Shape.unscoped.select(:type).distinct.pluck(:type)
    types.compact.each(&:constantize)
  end
end

11. Customizing Inflections

By default, Rails uses String#camelize to know which constant a given file or directory name should define. For example, posts_controller.rb should define PostsController because that is what "posts_controller".camelize returns.

It could be the case that some particular file or directory name does not get inflected as you want. For instance, html_parser.rb is expected to define HtmlParser by default. What if you prefer the class to be HTMLParser? There are a few ways to customize this.

The easiest way is to define acronyms:

ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.acronym "HTML"
  inflect.acronym "SSL"
end

Doing so affects how Active Support inflects globally. That may be fine in some applications, but you can also customize how to camelize individual basenames independently from Active Support by passing a collection of overrides to the default inflectors:

Rails.autoloaders.each do |autoloader|
  autoloader.inflector.inflect(
    "html_parser" => "HTMLParser",
    "ssl_error"   => "SSLError"
  )
end

That technique still depends on String#camelize, though, because that is what the default inflectors use as fallback. If you instead prefer not to depend on Active Support inflections at all and have absolute control over inflections, configure the inflectors to be instances of Zeitwerk::Inflector:

Rails.autoloaders.each do |autoloader|
  autoloader.inflector = Zeitwerk::Inflector.new
  autoloader.inflector.inflect(
    "html_parser" => "HTMLParser",
    "ssl_error"   => "SSLError"
  )
end

There is no global configuration that can affect said instances; they are deterministic.

You can even define a custom inflector for full flexibility. Please check the Zeitwerk documentation for further details.

11.1. Where Should Inflection Customization Go?

If an application does not use the once autoloader, the snippets above can go in config/initializers. For example, config/initializers/inflections.rb for the Active Support use case, or config/initializers/zeitwerk.rb for the other ones.

Applications using the once autoloader have to move or load this configuration from the body of the application class in config/application.rb, because the once autoloader uses the inflector early in the boot process.

12. Custom Namespaces

As we saw above, autoload paths represent the top-level namespace: Object.

Let's consider app/services, for example. This directory is not generated by default, but if it exists, Rails automatically adds it to the autoload paths.

By default, the file app/services/users/signup.rb is expected to define Users::Signup, but what if you prefer that entire subtree to be under a Services namespace? Well, with default settings, that can be accomplished by creating a subdirectory: app/services/services.

However, depending on your taste, that just might not feel right to you. You might prefer that app/services/users/signup.rb simply defines Services::Users::Signup.

Zeitwerk supports custom root namespaces to address this use case, and you can customize the main autoloader to accomplish that:

# config/initializers/autoloading.rb

# The namespace has to exist.
#
# In this example we define the module on the spot. Could also be created
# elsewhere and its definition loaded here with an ordinary `require`. In
# any case, `push_dir` expects a class or module object.
module Services; end

Rails.autoloaders.main.push_dir("#{Rails.root}/app/services", namespace: Services)

Rails < 7.1 did not support this feature, but you can still add this additional code in the same file and get it working:

# Additional code for applications running on Rails < 7.1.
app_services_dir = "#{Rails.root}/app/services" # has to be a string
ActiveSupport::Dependencies.autoload_paths.delete(app_services_dir)
Rails.application.config.watchable_dirs[app_services_dir] = [:rb]

Custom namespaces are also supported for the once autoloader. However, since that one is set up earlier in the boot process, the configuration cannot be done in an application initializer. Instead, please put it in config/application.rb, for example.

13. Autoloading and Engines

Engines run in the context of a parent application, and their code is autoloaded, reloaded, and eager loaded by the parent application. If the application runs in zeitwerk mode, the engine code is loaded by zeitwerk mode. If the application runs in classic mode, the engine code is loaded by classic mode.

When Rails boots, engine directories are added to the autoload paths, and from the point of view of the autoloader, there's no difference. Autoloaders' main inputs are the autoload paths, and whether they belong to the application source tree or to some engine source tree is irrelevant.

For example, this application uses Devise:

$ bin/rails runner 'pp ActiveSupport::Dependencies.autoload_paths'
[".../app/controllers",
 ".../app/controllers/concerns",
 ".../app/helpers",
 ".../app/models",
 ".../app/models/concerns",
 ".../gems/devise-4.8.0/app/controllers",
 ".../gems/devise-4.8.0/app/helpers",
 ".../gems/devise-4.8.0/app/mailers"]

If the engine controls the autoloading mode of its parent application, the engine can be written as usual.

However, if an engine supports Rails 6 or Rails 6.1 and does not control its parent applications, it has to be ready to run under either classic or zeitwerk mode. Things to take into account:

1. If classic mode would need a require_dependency call to ensure some constant is loaded at some point, write it. While zeitwerk would not need it, it won't hurt, it will work in zeitwerk mode too.

2. classic mode underscores constant names ("User" -> "user.rb"), and zeitwerk mode camelizes file names ("user.rb" -> "User"). They coincide in most cases, but they don't if there are series of consecutive uppercase letters as in "HTMLParser". The easiest way to be compatible is to avoid such names. In this case, pick "HtmlParser".

3. In classic mode, the file app/model/concerns/foo.rb is allowed to define both Foo and Concerns::Foo. In zeitwerk mode, there's only one option: it has to define Foo. In order to be compatible, define Foo.

14. Testing

14.1. Manual Testing

The task zeitwerk:check checks if the project tree follows the expected naming conventions and it is handy for manual checks. For example, if you're migrating from classic to zeitwerk mode, or if you're fixing something:

$ bin/rails zeitwerk:check
Hold on, I am eager loading the application.
All is good!

There can be additional output depending on the application configuration, but the last "All is good!" is what you are looking for.

14.2. Automated Testing

It is a good practice to verify in the test suite that the project eager loads correctly.

That covers Zeitwerk naming compliance and other possible error conditions. Please check the section about testing eager loading in the Testing Rails Applications guide.

15. Troubleshooting

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

The easiest way to do that is to include

Rails.autoloaders.log!

in 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 not yet available when config/application.rb executes. If you prefer to use the Rails logger, configure this setting in an initializer instead:

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

16. Rails.autoloaders

The Zeitwerk instances managing your application are available at

Rails.autoloaders.main
Rails.autoloaders.once

The predicate

Rails.autoloaders.zeitwerk_enabled?

is still available in Rails 7 applications, and returns true.