validates_presence_of (ActiveRecord::Validations::ClassMethods)

    # File lib/active_record/validations.rb, line 464
          def validates_presence_of(*attr_names)
            configuration = { :message => ActiveRecord::Errors.default_error_messages[:blank], :on => :save }
            configuration.update(attr_names.extract_options!)

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