application_controller.rb

require 'capybara'
require 'capybara/dsl'
require 'capybara/poltergeist'

Capybara.register_driver :poltergeist do |app|
  Capybara::Poltergeist::Driver.new(app, logger: nil, phantomjs_logger: nil)
end

class ApplicationController < ActionController::Base
  # Prevent CSRF attacks by raising an exception.
  # For APIs, you may want to use :null_session instead.
  protect_from_forgery with: :exception

  def index
    handle_search_engines!
  end

  private

  def handle_search_engines!
    if request.env["HTTP_USER_AGENT"] =~ /Googlebot|Bingbot|Baiduspider|iaskspider/
      webkit = Capybara::Session.new(:poltergeist)
      capybara_response = webkit.visit "#{request.scheme}://#{request.host_with_port}/#{params[:url]}"
      if capybara_response["status"] == "success"
        render text: webkit.source, layout: false
      end
      webkit.driver.quit
      return
    end
  end

end