Rails 6 ships with some pretty nice new features, the main one I'm pretty excited about is Action Mailbox, however writing RSpec tests for it requires a little bit of extra setup.

Add the support helper

First create a file in spec/support/mailbox.rb, inside it add:

# spec/support/mailbox.rb
require 'action_mailbox/test_helper'

RSpec.configure do |config|
  config.include ActionMailbox::TestHelper, type: :mailbox
end

This will include the ActionMailbox::TestHelper module, this adds methods like receive_inbound_email_from_source for use in your tests.

A sample RSpec Test

I've been using type: :mailbox and putting them in the spec/mailboxes/ folder, a sample looks like:

# spec/mailboxes/generic_emails_mailbox_spec.rb
require 'rails_helper'

RSpec.describe GenericEmailsMailbox, type: :mailbox do
  subject do
    receive_inbound_email_from_mail(
      from: 'from-address@example.com',
      to: 'to-address@example.com',
      subject: 'Sample Subject',
      body: "I'm a sample body"
    )
  end

  it do
    expect { subject }.to change(SomeModel, :count).by(1)
  end
end

Fixing "NotImplementedError"

If you hit an error that looks like:

NotImplementedError:
   Use a queueing backend to enqueue jobs in the future.

This means you're using Active Job in inline mode in your tests, which doesn't play nicely with with Active Mailbox. I fixed this by setting config.active_job.queue_adapter in my test environment to be :test.

# config/environments/test.rb
# Run Active Job in test mode.
config.active_job.queue_adapter = :test