save_changed_attribute

# File activerecord/lib/active_record/attribute_methods/dirty.rb, line 111
      def save_changed_attribute(attr, old_value)
        clear_changed_attributes_cache
        if attribute_changed_by_setter?(attr)
          clear_attribute_changes(attr) unless _field_changed?(attr, old_value)
        else
          set_attribute_was(attr, old_value) if _field_changed?(attr, old_value)
        end
      end