validates_exclusion_of (ActiveRecord::Validations::ClassMethods)

    # File lib/active_record/validations.rb, line 780
          def validates_exclusion_of(*attr_names)
            configuration = { :message => ActiveRecord::Errors.default_error_messages[:exclusion], :on => :save }
            configuration.update(attr_names.extract_options!)

            enum = configuration[:in] || configuration[:within]

            raise(ArgumentError, "An object with the method include? is required must be supplied as the :in option of the configuration hash") unless enum.respond_to?("include?")

            validates_each(attr_names, configuration) do |record, attr_name, value|
              record.errors.add(attr_name, configuration[:message] % value) if enum.include?(value)
            end
          end