write_attribute_with_dirty

# File activerecord/lib/active_record/dirty.rb, line 122
      def write_attribute_with_dirty(attr, value)
        attr = attr.to_s

        # The attribute already has an unsaved change.
        unless changed_attributes.include?(attr)
          old = clone_attribute_value(:read_attribute, attr)
          changed_attributes[attr] = old if field_changed?(attr, old, value)
        end

        # Carry on.
        write_attribute_without_dirty(attr, value)
      end