til/rails/halt-action-mailer-delivery-with-callback.md

Halt ActionMailer Delivery With Callback

ActionMailer supports callbacks, similar to ActiveRecord, like before_deliver and after_delivery. We can hook into the before_deliver callback to interrupt the delivery of an email that shouldn't go out.

Here's the scenario: you schedule a bunch of payment reminders to go out to your customers that still need to make their latest payment. Let's say the daily job that schedules all of these reminders runs in the middle of the night, but schedules the emails to land in inboxes at a more reasonable time, like 10am. Between the time that the email is scheduled and it gets processed for delivery, a customer makes their payment. In that case, we no longer want to send that person an email reminder.

To handle this scenario, we can have a before_deliver callback that checks the user's balance and raises :abort to halt the callback execution chain, effectively preventing the email from going out. We can even scope the callback to just the actions we care about using the if option and checking the action_name.

class UserMailer < ApplicationMailer
  before_deliver :abort_if_payment_is_current,
    if: -> { action_name.in?(%w[payment_reminder past_due_invoice]) }

  def payment_reminder
    # ...
  end

  def past_due_invoice
    # ...
  end

  private

  def abort_if_payment_is_current
    if @user.check_latest_balance.zero?
      raise :abort
    end
  end
end

See Action Mailer Callbacks for more details.