calculate_changes_from_defaults

# File activerecord/lib/active_record/attribute_methods/dirty.rb, line 84
      def calculate_changes_from_defaults
        @changed_attributes = nil
        self.class.column_defaults.each do |attr, orig_value|
          set_attribute_was(attr, orig_value) if _field_changed?(attr, orig_value)
        end
      end