Blacklist known-incompatible browsers and versions in Rails

gistfile1.rb

# assumes UserAgent gem is available
# https://rubygems.org/gems/useragent

class ApplicationController < ActionController::Base

  # blacklist recognized browser with lower versions than those below
  Browser = Struct.new(:browser, :version)
  BROWSERS = [
    Browser.new("Chrome", "3.0"),
    Browser.new("Firefox", "3.0"),
    Browser.new("Internet Explorer", "8.0"),
    Browser.new("Opera", "9.5"),
    Browser.new("Safari", "4.0")
  ]

  before_filter :check_browser, except: [ :upgrade_browser ]

  def check_browser
    if request.format == :html && 
      !current_user &&
      (user_agent = UserAgent.parse(request.user_agent)) &&
      user_agent.size > 0 &&
      user_agent.version &&
      BROWSERS.any? {|browser| user_agent.browser == browser[0] } && 
      BROWSERS.none? { |browser| user_agent >= browser }
        redirect_to upgrade_browser_path
      end
    end
  end