    # File lib/active_record/validations.rb, line 400
          def validates_presence_of(*attr_names)
            configuration = { :message => ActiveRecord::Errors.default_error_messages[:blank], :on => :save }
            configuration.update(attr_names.pop) if attr_names.last.is_a?(Hash)

            # can't use validates_each here, because it cannot cope with nonexistent attributes,
            # while errors.add_on_empty can
            attr_names.each do |attr_name|
              send(validation_method(configuration[:on])) do |record|
                unless configuration[:if] and not evaluate_condition(configuration[:if], record)
                  record.errors.add_on_blank(attr_name,configuration[:message])
                end
              end
            end
          end