Offline template renderer to use with rake tasks (or any background worker)

offline_template.rb

    class OfflineTemplate < AbstractController::Base
      include AbstractController::Logger
      include AbstractController::Rendering
      include AbstractController::Layouts
      include AbstractController::Helpers
      include AbstractController::Translation
      include AbstractController::AssetPaths
      include ActionDispatch::Routing::UrlFor
      include Rails.application.routes.url_helpers
      Rails.application.routes.default_url_options = { :host => 'www.yoursite.com' }

      helper ApplicationHelper

      helper_method :protect_against_forgery?

      # configure the different paths correctly
      def initialize(*args)
        super()
        lookup_context.view_paths = Rails.root.join('app', 'views')
        config.javascripts_dir = Rails.root.join('public', 'javascripts')
        config.stylesheets_dir = Rails.root.join('public', 'stylesheets')
        config.assets_dir = Rails.root.join('public')
      end

      # we are not in a browser, no need for this
      def protect_against_forgery?
        false
      end

      # so that your flash calls still work
      def flash
        {}
      end

      def params
        {}
      end

      # same asset host as the controllers
      self.asset_host = ActionController::Base.asset_host
    end

usage.rb

template = OfflineTemplate.new
pdf_page = template.render_to_string partial: 'partial', layout: "pdf", formats: :html, locals: {:@user => user}

kit = PDFKit.new(pdf_page)
kit.stylesheets << "#{Rails.root}/app/assets/stylesheets/pdf/pdf.css"

pdf = open(kit.to_file(temp_dir + user.id.to_s + '_tmp.pdf'))