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Classic to Zeitwerk HOWTO

This guide documents how to migrate Rails applications from classic to zeitwerk mode.

After reading this guide, you will know:

1 What are classic and zeitwerk Modes?

From the very beginning, and up to Rails 5, Rails used an autoloader implemented in Active Support. This autoloader is known as classic and is still available in Rails 6.x. Rails 7 does not include this autoloader anymore.

Starting with Rails 6, Rails ships with a new and better way to autoload, which delegates to the Zeitwerk gem. This is zeitwerk mode. By default, applications loading the 6.0 and 6.1 framework defaults run in zeitwerk mode, and this is the only mode available in Rails 7.

2 Why Switch from classic to zeitwerk?

The classic autoloader has been extremely useful, but had a number of issues that made autoloading a bit tricky and confusing at times. Zeitwerk was developed to address this, among other motivations.

When upgrading to Rails 6.x, it is highly encouraged to switch to zeitwerk mode because it is a better autoloader, classic mode is deprecated.

Rails 7 ends the transition period and does not include classic mode.

3 I am Scared

Don't :).

Zeitwerk was designed to be as compatible with the classic autoloader as possible. If you have a working application autoloading correctly today, chances are the switch will be easy. Many projects, big and small, have reported really smooth switches.

This guide will help you change the autoloader with confidence.

If for whatever reason you find a situation you don't know how to resolve, don't hesitate to open an issue in rails/rails and tag @fxn.

4 How to Activate zeitwerk Mode

4.1 Applications running Rails 5.x or Less

In applications running a Rails version previous to 6.0, zeitwerk mode is not available. You need to be at least in Rails 6.0.

4.2 Applications running Rails 6.x

In applications running Rails 6.x there are two scenarios.

If the application is loading the framework defaults of Rails 6.0 or 6.1 and it is running in classic mode, it must be opting out by hand. You have to have something similar to this:

# config/application.rb
config.load_defaults 6.0
config.autoloader = :classic # DELETE THIS LINE

As noted, just delete the override, zeitwerk mode is the default.

On the other hand, if the application is loading old framework defaults you need to enable zeitwerk mode explicitly:

# config/application.rb
config.load_defaults 5.2
config.autoloader = :zeitwerk

4.3 Applications Running Rails 7

In Rails 7 there is only zeitwerk mode, you do not need to do anything to enable it.

Indeed, in Rails 7 the setter config.autoloader= does not even exist. If config/application.rb uses it, please delete the line.

5 How to Verify The Application Runs in zeitwerk Mode?

To verify the application is running in zeitwerk mode, execute

bin/rails runner 'p Rails.autoloaders.zeitwerk_enabled?'

If that prints true, zeitwerk mode is enabled.

6 Does my Application Comply with Zeitwerk Conventions?

Once zeitwerk mode is enabled, please run:

bin/rails zeitwerk:check

A successful check looks like this:

% 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.

If there's any file that does not define the expected constant, the task will tell you. It does so one file at a time, because if it moved on, the failure loading one file could cascade into other failures unrelated to the check we want to run and the error report would be confusing.

If there's one constant reported, fix that particular one and run the task again. Repeat until you get "All is good!".

Take for example:

% bin/rails zeitwerk:check
Hold on, I am eager loading the application.
expected file app/models/vat.rb to define constant Vat

VAT is an European tax. The file app/models/vat.rb defines VAT but the autoloader expects Vat, why?

6.1 Acronyms

This is the most common kind of discrepancy you may find, it has to do with acronyms. Let's understand why do we get that error message.

The classic autoloader is able to autoload VAT because its input is the name of the missing constant, VAT, invokes underscore on it, which yields vat, and looks for a file called vat.rb. It works.

The input of the new autoloader is the file system. Give the file vat.rb, Zeitwerk invokes camelize on vat, which yields Vat, and expects the file to define the constant Vat. That is what the error message says.

Fixing this is easy, you only need to tell the inflector about this acronym:

# config/initializers/inflections.rb
ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.acronym "VAT"
end

Doing so affects how Active Support inflects globally. That may be fine, but if you prefer you can also pass overrides to the inflector used by the autoloader:

# config/initializers/zeitwerk.rb
Rails.autoloaders.each do |autoloader|
  autoloader.inflector.inflect("vat" => "VAT")
end

With that in place, the check passes!

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

Once all is good, it is recommended to keep validating the project in the test suite. The section Check Zeitwerk Compliance in the Test Suite explains how to do this.

6.2 Concerns

You can autoload and eager load from a standard structure with concerns subdirectories like

app/models
app/models/concerns

By default, app/models/concerns belongs to the autoload paths and therefore it is assumed to be a root directory. So, by default, app/models/concerns/foo.rb should define Foo, not Concerns::Foo.

If your application uses Concerns as namespace, you have two options:

  1. Remove the Concerns namespace from those classes and modules and update client code.
  2. Leave things as they are by removing app/models/concerns from the autoload paths:
  # config/initializers/zeitwerk.rb
  ActiveSupport::Dependencies.
    autoload_paths.
    delete("#{Rails.root}/app/models/concerns")

6.3 Having app in the autoload paths

Some projects want something like app/api/base.rb to define API::Base, and add app to the autoload paths to accomplish that.

Since Rails adds all subdirectories of app to the autoload paths automatically (with a few exceptions like directories for assets), we have another situation in which there are nested root directories, similar to what happens with app/models/concerns. That setup no longer works as is.

However, you can keep that structure, just delete app/api from the autoload paths in an initializer:

# config/initializers/zeitwerk.rb
ActiveSupport::Dependencies.
  autoload_paths.
  delete("#{Rails.root}/app/api")

6.4 Autoloaded Constants and Explicit Namespaces

If a namespace is defined in a file, as Hotel is here:

app/models/hotel.rb         # Defines Hotel.
app/models/hotel/pricing.rb # Defines Hotel::Pricing.

the Hotel constant has to be set using the class or module keywords. For example:

class Hotel
end

is good.

Alternatives like

Hotel = Class.new

or

Hotel = Struct.new

won't work, child objects like Hotel::Pricing won't be found.

This restriction only applies to explicit namespaces. Classes and modules not defining a namespace can be defined using those idioms.

6.5 One file, one constant (at the same top-level)

In classic mode you could technically define several constants at the same top-level and have them all reloaded. For example, given

# app/models/foo.rb

class Foo
end

class Bar
end

while Bar could not be autoloaded, autoloading Foo would mark Bar as autoloaded too.

This is not the case in zeitwerk mode, you need to move Bar to its own file bar.rb. One file, one top-level constant.

This affects only to constants at the same top-level as in the example above. Inner classes and modules are fine. For example, consider

# app/models/foo.rb

class Foo
  class InnerClass
  end
end

If the application reloads Foo, it will reload Foo::InnerClass too.

6.6 Globs in config.autoload_paths

Beware of configurations that use wildcards like

config.autoload_paths += Dir["#{config.root}/extras/**/"]

Every element of config.autoload_paths should represent the top-level namespace (Object). That won't work.

To fix this, just remove the wildcards:

config.autoload_paths << "#{config.root}/extras"

6.7 Spring and the test Environment

Spring reloads the application code if something changes. In the test environment you need to enable reloading for that to work:

# config/environments/test.rb
config.cache_classes = false

Otherwise you'll get this error:

reloading is disabled because config.cache_classes is true

This has no performance penalty.

6.8 Bootsnap

Please make sure to depend on at least Bootsnap 1.4.4.

7 Check Zeitwerk Compliance in the Test Suite

The task zeitwerk:check is handy while migrating. Once the project is compliant, it is recommended to automate this check. In order to do so, it is enough to eager load the application, which is all zeitwerk:check does, indeed.

7.1 Continuous Integration

If your project has continuous integration in place, it is a good idea to eager load the application when the suite runs there. If the application cannot be eager loaded for whatever reason, you want to know in CI, better than in production, right?

CIs typically set some environment variable to indicate the test suite is running there. For example, it could be CI:

# config/environments/test.rb
config.eager_load = ENV["CI"].present?

Starting with Rails 7, newly generated applications are configured that way by default.

7.2 Bare Test Suites

If your project does not have continuous integration, you can still eager load in the test suite by calling Rails.application.eager_load!:

7.2.1 minitest
require "test_helper"

class ZeitwerkComplianceTest < ActiveSupport::TestCase
  test "eager loads all files without errors" do
    assert_nothing_raised { Rails.application.eager_load! }
  end
end
7.2.2 RSpec
require "rails_helper"

RSpec.describe "Zeitwerk compliance" do
  it "eager loads all files without errors" do
    expect { Rails.application.eager_load! }.not_to raise_error
  end
end

8 Delete any require calls

In my experience, projects generally do not do this. But I've seen a couple, and have heard of a few others.

In Rails application you use require exclusively to load code from lib or from 3rd party like gem dependencies or the standard library. Never load autoloadable application code with require. See why this was a bad idea already in classic here.

require "nokogiri" # GOOD
require "net/http" # GOOD
require "user"     # BAD, DELETE THIS (assuming app/models/user.rb)

Please delete any require calls of that type.

9 New Features You Can Leverage

9.1 Delete require_dependency calls

All known use cases of require_dependency have been eliminated with Zeitwerk. You should grep the project and delete them.

If your application uses Single Table Inheritance, please see the Single Table Inheritance section of the Autoloading and Reloading Constants (Zeitwerk Mode) guide.

9.2 Qualified Names in Class and Module Definitions Are Now Possible

You can now robustly use constant paths in class and module definitions:

# Autoloading in this class body matches Ruby semantics now.
class Admin::UsersController < ApplicationController
  # ...
end

A gotcha to be aware of is that, depending on the order of execution, the classic autoloader could sometimes be able to autoload Foo::Wadus in

class Foo::Bar
  Wadus
end

That does not match Ruby semantics because Foo is not in the nesting, and won't work at all in zeitwerk mode. If you find such corner case you can use the qualified name Foo::Wadus:

class Foo::Bar
  Foo::Wadus
end

or add Foo to the nesting:

module Foo
  class Bar
    Wadus
  end
end

9.3 Thread-safety Everywhere

In classic mode, constant autoloading is not thread-safe, though Rails has locks in place for example to make web requests thread-safe.

Constant autoloading is thread-safe in zeitwerk mode. For example, you can now autoload in multi-threaded scripts executed by the runner command.

9.4 Eager Loading and Autoloading are Consistent

In classic mode, if app/models/foo.rb defines Bar, you won't be able to autoload that file, but eager loading will work because it loads files recursively blindly. This can be a source of errors if you test things first eager loading, execution may fail later autoloading.

In zeitwerk mode both loading modes are consistent, they fail and err in the same files.

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