storefront.rb

class Storefront < ActiveRecord::Base
    extend WebsiteValidation
    validates_website :website
end

website_validation.rb

module WebsiteValidation

  def validates_website(column_name)
    validates do
      unless __send__(column_name).blank? || __send__(column_name) =~ /^http:\/\/|^https:\/\//
        __send__("#{column_name}=", "http://#{website}")
      end
    end
  end

end