Rails 6 & deliver_later doesn't affect ActionMailer::Base.deliveries - delayed-job

Question

After upgrading to Rails 6 I am noticing that default mailer's .deliver_later is not working the same as in Rails 5.
Configuration:
config.active_job.queue_adapter = :inline
When running Mailer.register_email(...).deliver_later - nothing is stored in ActionMailer::Base.deliveries. This array gets filled if I run perform_enqueued_jobs - it seams like queue_adapter = :inline doesn't work the way I expect it to work.
If I run Mailer.send(...).deliver_now then ActionMailer::Base.deliveries has proper value in it.
Any idea why this is happening and how to solve this?

Answer 1

I had same problem in my tests. A search on the Internet yielded nothing, so I started experimenting.
I tried wrapping the call method of sending mail in
assert_emails 1 do
Mailer.register_email(...).deliver_later
end
After that, ActionMailer::Base.deliveries populated correctly.

Answer 2

If the exact number of emails could easily change, this is another option:
assert_changes 'enqueued_jobs.size' do
# Some code that sends email with deliver_later
end
This allows you to test that emails were sent but it disregards the exact number (which is a limitation of the asserts_emails method - other than this, the asserts_emails method is great).
I found that the enqueued_jobs method is very helpful in testing anything background jobs, including deliver_later
NOTE: the above example only checks that the enqueued jobs list was changed. If you want to be more specific and check that the queue was changed with emails, you should do this:
assert_changes 'enqueued_jobs.select {|job| job["job_class"] == "ActionMailer::MailDeliveryJob"}.size' do
# Some code that sends email with deliver_later
end

Answer 3

The issue
The issue lies in two new lines of code added to Rails 6 (line 1 & line 2),
where basically, the callback before_setup defined here (in RSpec) and here (in Minitest) gets overridden (by this), thus forcing the queue_adapter to be the test adapter instead of the one defined by config.active_job.queue_adapter.
Workaround
So in order to use the queue_adapter defined by config.active_job.queue_adapter and therefore restore the Rails 5 behaviour we can do something like the below.
# spec/support/active_job/test_helper.rb
module ActiveJob
module TestHelper
def before_setup
super
end
end
end