Defer mailer include via ActiveSupport.on_load(:action_mailer)#926
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scambra merged 1 commit intoMay 22, 2026
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The to_prepare callback was forcing Devise.mailer (a String#constantize) to resolve at app initialization time, which loaded ActionMailer::Base and — via the default ActionMailer::Base inheritance chain pulling in MailDeliveryJob — ActiveJob::Base before app initialization completed. Rails edge's guard_load_hooks support (rails/rails#56201) reports both loads as 'loaded before application initialization' on every boot. Wrapping the include in on_load(:action_mailer) keeps the include happening on every reload (the on_load callback runs immediately when re-registered after :action_mailer has fired) without forcing the load during the Rails finisher initializer.
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Fixes #925.
config.to_preparein the Engine forcesDevise.mailerto constantize during the Rails finisher initializer, which in turn loadsActionMailer::Base(and, viaMailDeliveryJob,ActiveJob::Base) beforeRails.application.initialized?flips. Rails edge's newguard_load_hookssupport (rails/rails#56201) reports this as a premature load-hook for both:action_mailerand:active_jobon every boot.This change wraps the body of the
to_prepareblock inActiveSupport.on_load(:action_mailer):to_prepareregisters anon_load(:action_mailer)hook. Nothing loadsActionMailer::Baseyet. When something later resolves a mailer (sending an invitation, etc.), the hook fires and the include runs against the loaded mailer class. App init has already completed, so no warning.to_preparecycles (dev reloads).:action_mailerhas already fired, so the freshly-registeredon_loadblock runs immediately, re-applying the include on the reloadedDevise.mailerclass — same behaviour as before.No functional change for stable Rails users; just silences the boot warnings on Rails edge and saves a small amount of boot time by deferring the mailer load.
I didn't add a regression test — wasn't sure how to best exercise railtie load order in the existing harness. Happy to add one if you'd like, or to take suggestions on the right approach.