Add Halt ActionMailer Delivery With Callback as a Rails TIL

README.md

@@ -10,7 +10,7 @@ working across different projects via [VisualMode](https://www.visualmode.dev/).
 
 For a steady stream of TILs, [sign up for my newsletter](https://visualmode.kit.com/newsletter).
 
-_1795 TILs and counting..._
+_1796 TILs and counting..._
 
 See some of the other learning resources I work on:
 
@@ -1175,6 +1175,7 @@ If you've learned something here, support my efforts writing daily TILs by
 - [Get The Column Names For A Model](rails/get-the-column-names-for-a-model.md)
 - [Get The Current Time](rails/get-the-current-time.md)
 - [Grab A Random Record From The Database](rails/grab-a-random-record-from-the-database.md)
+- [Halt ActionMailer Delivery With Callback](rails/halt-action-mailer-delivery-with-callback.md)
 - [Handle Named Arguments In A Rake Task](rails/handle-named-arguments-in-a-rake-task.md)
 - [Hash Slicing](rails/hash-slicing.md)
 - [Ignore Poltergeist JavaScript Errors](rails/ignore-poltergeist-javascript-errors.md)

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

@@ -0,0 +1,46 @@
+# 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`.
+
+```ruby
+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](https://guides.rubyonrails.org/action_mailer_basics.html#action-mailer-callbacks)
+for more details.