Force Rails to render flash messages for XHR responses

render_flash_now_for_xhr.rb

# To display flash messages inside XHR responses,
# place this file in app/controllers/concerns/
# then include it in ApplicationController

module RenderFlashNowForXhr
  extend ActiveSupport::Concern

  private

  # Flash messages are not directly available for XHR requests
  # This concern rewrites `render notice: message` to `flash.now.notice = message; render`
  def render(*args)
    if request.xhr? && args.length.positive?
      options = args.find { |arg| arg.is_a? Hash }
      return super if options.nil?

      [:alert, :error, :notice, :success].each do |type|
        flash.now[type] = options[type] if type.in? options
      end
    end
    super
  end
end