gistfile1.rb

  ##
  # Validates a URL
  #
  # If the URI library can parse the value, and the scheme is valid
  # then we assume the url is valid
  #
  

require 'addressable/uri'
require 'net/http'

module URLValidator
  class ActiveRecord::Base
    def validate_url(attribute)
      value = self.send(attribute)
      begin
        uri = Addressable::URI.parse(value)
        if !["http","https","ftp"].include?(uri.scheme)
          raise Addressable::URI::InvalidURIError
        end
      rescue Addressable::URI::InvalidURIError
        # URI is unparseable, manually try it
        begin # check header response
          case Net::HTTP.get_response(Addressable::URI.parse(value))
              when Net::HTTPSuccess then true
              else (self.errors[attribute] << "Not a live URL")
          end
        rescue # Recover on DNS failures..
          self.errors[attribute] << "Invalid URL"
        end
      end
    end
  end
end

To use:

class Foo < ActiveRecord::Base
include URLValidator
validate_url :website
end