instance method save_changed_attribute

# File activerecord/lib/active_record/enum.rb, line 142
              def save_changed_attribute(attr_name, old)
                if (mapping = self.class.defined_enums[attr_name.to_s])
                  value = _read_attribute(attr_name)
                  if attribute_changed?(attr_name)
                    if mapping[old] == value
                      clear_attribute_changes([attr_name])
                    end
                  else
                    if old != value
                      set_attribute_was(attr_name, mapping.key(old))
                    end
                  end
                else
                  super
                end
              end