save_changed_attribute

# File activerecord/lib/active_record/attribute_methods/dirty.rb, line 71
      def save_changed_attribute(attr, value)
        # The attribute already has an unsaved change.
        if attribute_changed?(attr)
          old = changed_attributes[attr]
          changed_attributes.delete(attr) unless _field_changed?(attr, old, value)
        else
          old = clone_attribute_value(:read_attribute, attr)
          changed_attributes[attr] = old if _field_changed?(attr, old, value)
        end
      end