How to set a custom preview if mounted into subdirectory

[dssjoblom]

Steps to reproduce

Configure Rails with a relative URL root. https://guides.rubyonrails.org/configuring.html#deploy-to-a-subdirectory-relative-url-root

Expected behavior

Alchemy should only use URLs with the specified root.

Actual behavior

The preview window attempts to access the non-relative root /. For example, assume RAILS_RELATIVE_URL_ROOT is "/subdir" and Alchemy is mounted on "cms". The preview will then attempt to load e.g. /cms/admin/pages/1, which doesn't exist, while it should instead load /subdir/cms/admin/pages/1. It is possible this same issue is present in other parts of the code as well.

System configuration

• Alchemy Version: 6.0.0-rc6
• Rails Version: 6.1.4.7

[tvdeyen]

You can register your own preview url - even several sources admins can select from are possible.

Preview window URL configuration

By default Alchemy uses its internal page preview renderer,
but you can configure it to be any URL instead.

Basic Auth is supported.

Example config/alchemy/config.yml

preview:
  host: https://www.my-static-site.com
  auth:
    username: <%= ENV["BASIC_AUTH_USERNAME"] %>
    password: <%= ENV["BASIC_AUTH_PASSWORD"] %>

Preview config per site is supported as well.

Example config/alchemy/config.yml

preview:
  My site name:
    host: https://www.my-static-site.com
    auth:
      username: <%= ENV["BASIC_AUTH_USERNAME"] %>
      password: <%= ENV["BASIC_AUTH_PASSWORD"] %>

Define page preview sources

A preview source is a Ruby class returning an URL
that is used as source for the preview frame in the
admin UI.

Example

# lib/acme/preview_source.rb
class Acme::PreviewSource < Alchemy::Admin::PreviewUrl
  def url_for(page)
    if page.site.name == "Next"
      "https://user:#{ENV['PREVIEW_HTTP_PASS']}@next.acme.com"
    else
      "https://www.acme.com"
    end
  end
end

# config/initializers/alchemy.rb
require "acme/preview_source"
Alchemy.preview_sources << Acme::PreviewSource

# config/locales/de.yml
de:
  activemodel:
    models:
      acme/preview_source: Acme Vorschau

[dssjoblom]

Thanks for the pointers, @tvdeyen. I was able to locate the problem, but it seems like it is actually more or less a Rails bug. Engines don't get the right paths from the URL helpers by default. In case anyone else runs into this, I was able to work around this by doing:

if ENV['RAILS_RELATIVE_URL_ROOT']
  # Required for Alchemy CMS to generate correct URLs
  Rails.application.routes.default_url_options[:script_name] =
    ENV['RAILS_RELATIVE_URL_ROOT']
end

This code should be inserted before the routes are drawn. I also opened an issue in Rails for this, rails/rails#44919.