How do I fix my UserMailer / Devise::Mailer to satisfy deprecation warning in Rails 6?

Question

I customize the Devise default mailers by inheriting them in UserMailer but this had triggered a deprecation warning. I don't really understand what is it telling me.

DEPRECATION WARNING: Initialization autoloaded the constants Devise::Mailer, ApplicationHelper, and UserMailer.

Being able to do this is deprecated. Autoloading during initialization is going
to be an error condition in future versions of Rails.

Reloading does not reboot the application, and therefore code executed during
initialization does not run again. So, if you reload Devise::Mailer, for example,
the expected changes won't be reflected in that stale Class object.

These autoloaded constants have been unloaded.

# devise.rb

...
config.mailer = UserMailer
config.parent_mailer = 'ApplicationMailer'
...

# user_mailer.rb

class UserMailer < Devise::Mailer
  helper :application
  include Devise::Controllers::UrlHelpers
  ...
end

Answer 1

To prevent that DEPRECATION warning, you could require those 3 classes (Devise::Mailer, ApplicationHelper, and UserMailer) in your config/application.rb

I ran into a very similar error message: DEPRECATION WARNING: Initialization autoloaded the constants ApplicationRecord, Status, and Task.

I fixed it like this:

# config/application.rb
. . .
module MyApp
  class Application < Rails::Application
    require "#{Rails.root}/app/models/application_record"
    require "#{Rails.root}/app/models/concerns/status"
    require "#{Rails.root}/app/models/task"
. . .

The reason you're seeing that DEPRECATION warning is because Rails 6 defaults to using Zeitwerk to load dependencies and the classes Rails is complaining about, unless fixed, will very likely have problems loading properly in future versions of Rails.

For more info see: https://guides.rubyonrails.org/autoloading_and_reloading_constants.html