I recently needed to implement some tests for the SandboxEmailInterceptor Intializer pattern suggested by the rails guide. I found two methods to test the class but neither of them are perfect.

You can test the functionality of the interceptor

test 'it should redirect email when interceptor is run' do
  SandboxEmailInterceptor.delivering_email(@email)
  assert_equal @email.to, ['sandbox@example.com']
end

However, passing this test doesn’t mean your emails will be intercepted.

Stub the environment and load the initializer

test 'it sends all emails to staging@donorsiblingregistry.com in staging' do
  Rails.stub(:env, ActiveSupport::StringInquirer.new('staging')) do
    load 'config/initializers/sandbox_email_interceptor.rb'

    @email.deliver

    last_email_sent = ActionMailer::Base.deliveries.last
    assert_equal last_email_sent.to, ['sandbox@example.com']
  end
end

This gets us closer but if your initializers aren’t running, for example then this test will still be inadequate. I haven’t found a better solution than this one so please comment if you know of one.