Rails admin with Sorcery

I'm trying to install the Rails Admin Gem using Sorcery for authentication instead of Devise.

Rails admin does provide a hook that you can use to attach your own authentication method. Here is the example they provide in their docs (using warden):

config.authenticate_with do
  warden.authenticate! :scope => :admin
end
config.current_user_method { current_admin }

I'm guessing that inside the block I need to reference the before_filter that Sorcery uses to authenticate users, which would be require_login.

However, when I try that and I try to visit /admin when logged out, I get a routing error:

No route matches {:action=>"new", :controller=>"sessions"}

This probably happens because I am being redirected within the engine rather than in the main app.

How can I set this up correctly?

# config/initializers/rails_admin.rb
RailsAdmin.config do |config|
  config.authenticate_with do
    # Use sorcery's before filter to auth users
    require_login
  end
end

# app/controllers/application_controller.rb
class ApplicationController
  # Overwrite the method sorcery calls when it
  # detects a non-authenticated request.
  def not_authenticated
    # Make sure that we reference the route from the main app.
    redirect_to main_app.login_path
  end
end

#config/initializers/rails_admin.rb
RailsAdmin.config do |config|
  ...
  config.parent_controller = 'ApplicationController'
end