Preview Emails in ActiveAdmin / Rails 5.1

email_previews.rb

-# app/admin/email_previews.rb
# frozen_string_literal: true

ActiveAdmin.register_page 'Email Previews' do
  menu parent: 'dashboard', priority: 10

  content do
    div 'Select a mailer from the sidebar.'
  end

  sidebar 'Mail Previews' do
    Dir['spec/mailer_previews/**/*_preview.rb'].each do |preview_path|
      preview_mailer = File.basename(preview_path, '.rb')
      mailer = preview_mailer.chomp('_preview')
      div { strong { mailer } }
      preview_mailer_class = preview_mailer.camelize.constantize
      mails = preview_mailer_class.public_instance_methods(false).map(&:to_s).sort
      mails.each do |mail|
        div { link_to mail, {action: :preview, mailer: mailer, mail: mail} }
      end
      br
    end
  end

  page_action :preview
end

preview.html.slim

-# app/views/admin/email_previews/preview.html.slim
iframe src="/admin/mailers/#{params[:mailer]}/#{params[:mail]}" style="width: 100%; height: 1000px; border: 2px solid lightgrey;"

preview_emails.rb

# config/initializers/preview_emails.rb
# frozen_string_literal: true

Rails.application.configure do
  config.action_mailer.show_previews = true
  config.action_mailer.preview_path ||= Rails.root.join('spec/mailer_previews')
  config.autoload_paths += [config.action_mailer.preview_path]

  routes.append do
    get '/admin/mailers' => 'rails/mailers#index'
    get '/admin/mailers/*path' => 'rails/mailers#preview'
  end
end

class Rails::MailersController
  before_action :require_admin

  private

  def require_admin
    redirect_to root_path unless current_user&.admin?
  end
end