Previewing Mailers on non-development tiers - ruby-on-rails

Question

I have several mailer previews under spec/mailer/previews. On development I can view all the previews under /rails/mailers/. However by default this functionality does not exist on other environments.
I wanted to enable it on the staging environment and took a queue from this post here.
I made the following changes -
config/routes.rb
# Add the routes manually
if Rails.env.staging?
get "/rails/mailers" => "rails/mailers#index"
get "/rails/mailers/*path" => "rails/mailers#preview"
end
config/environments/staging.rb
Rails.application.configure do
# Define the mailer preview path
config.action_mailer.preview_path = "spec/mailers/previews"
# Specifically add that path and all files under it to the autoload paths
config.autoload_paths = Dir["#{config.root}/#{config.action_mailer.preview_path}/**"]
end
class ::Rails::MailersController
include Rails.application.routes.url_helpers
# Override the method just for this controller so `MailersController` thinks
# all requests are local.
def local_request?
true
end
end
However on staging, I'm getting the following error when trying to load the /rails/mailers page -
LoadError (No such file to load -- spec/mailers/previews/admin_mailer_preview):
The odd thing is... that file definitely exists. And when I check the autoload paths on staging that file is definitely in the Array/list.
Any thoughts on what might be happening here, or how else I should go about exposing that endpoint?
Thanks!

Answer 1

Having consider_all_requests_local = true or patching local_request? can be security issue. Here is solution we use, it uses authentication to allow only admins to access previews:
# in any enviroment
config.action_mailer.show_previews = true
# in initializers/mailer_previews.rb
# (use your own authentication methods)
# Allow admins to see previews if show_previews enabled.
# It does not affect dev env, as this setting is nil there.
if Rails.application.config.action_mailer.show_previews
Rails::MailersController.prepend_before_action do
authenticate_user!
head :forbidden unless current_user.admin?
end
end
# If you use rspec-rails, it makes rails use spec/mailers/previews
# as previews path. Most likely you don't have rspec-rails on
# staging/production, so need to add this to application.rb:
#
# Make previews available in environments without rspec-rails.
config.action_mailer.preview_path = Rails.root.join('spec', 'mailers', 'previews')
# Bonus: specs. Update with your `sign_in` implementation
# and have `config.action_mailer.show_previews = true` in test.rb
RSpec.describe Rails::MailersController do
shared_examples 'visible to admin only' do
it { should redirect_to new_user_session_path }
context 'for signed in user' do
sign_in { create(:user) }
it { should be_forbidden }
context 'admin' do
let(:current_user) { create(:user, admin: true) }
it { should be_ok }
end
end
end
describe '#index' do
subject { get('/rails/mailers') && response }
include_examples 'visible to admin only'
end
describe '#preview' do
subject { get('/rails/mailers/devise/mailer') && response }
include_examples 'visible to admin only'
end
end

Answer 2

It depends on what Rails version are you running, but if you are on 4.2+ adding these lines to staging.rb should help:
config.action_mailer.show_previews = true
config.consider_all_requests_local = true

Answer 3

Another option would be to use a service like https://mailtrap.io/
and also get some more interesting information about the email such as spam and responsiveness analysis - I find it to be the best option for my staging environment.