How to render an email preview in Rails

I needed to render an email preview for users. Rails has an email preview system, but that's for development.

Searching didn't turn up much. Except for the development options. So I dug into the code. Something I find myself doing more of.

There is an ActionMailer::Preview class. Turns out that's also for the development process. But there is a private method that does the work.

I pulled that code out and put into a controller.

...
  @email = UserMailer.invite_existing_letter_recipient(@letter, current_user)
  ActionMailer::Base.preview_interceptors.each do |interceptor|
    interceptor.previewing_email(@email)
  end
  @preview = @email.html_part.decoded
...

And in the view.

  <%= @preview.html_safe %>

Calling the interceptors does things like fix the inline images and inline the CSS. I'm using Inky. You might be using MJML. And those work by intercepting.

Lastly, I need to render it in an iFrame so the site styles don't interfere with the email.