Add Test If deliver_later Is Called For A Mailer as a rails til

rails/test-if-deliver-later-is-called-for-a-mailer.md

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+# Test If deliver_later Is Called For A Mailer
+
+There are many ways to test in your controller whether emails are going out. A
+concise and quick way to check is just to see if a `deliver_later` happened.
+
+Depending on how your test environment is configured, this could look one of
+two ways.
+
+If you have your `queue_adapter` set to
+[`:inline`](https://api.rubyonrails.org/classes/ActiveJob/QueueAdapters/InlineAdapter.html),
+then a `deliver_later` will happen synchronously. So, the email will
+immediately end up in the `deliveries` box.
+
+```ruby
+expect {
+  post :password_reset, params: valid_params
+}.to change { ActionMailer::Base.deliveries.count }.by(1)
+```
+
+The behavior is a bit different if your `queue_adapter` is set to something
+like
+[`:test`](https://api.rubyonrails.org/classes/ActiveJob/QueueAdapters/TestAdapter.html).
+In this case, the email is going to be queued in the app's job queue. Since it
+is not immediately being sent, the expectation will have to be about the job
+queue instead.
+
+```ruby
+expect {
+  post :password_reset, params: valid_params
+}.to have_enqueued_job(ActionMailer::DeliveryJob)
+```
+
+We can even dig into more specifics like this:
+
+```ruby
+expect {
+  post :password_reset, params: valid_params
+}.to have_enqueued_job(ActionMailer::DeliveryJob)
+  .with('UserMailer', 'password_reset', 'deliver_later', Integer)
+```