instance method _field_changed?

# File activerecord/lib/active_record/attribute_methods/dirty.rb, line 80
      def _field_changed?(attr, old, value)
        if column = column_for_attribute(attr)
          if column.number? && (changes_from_nil_to_empty_string?(column, old, value) ||
                                changes_from_zero_to_string?(old, value))
            value = nil
          else
            value = column.type_cast(value)
          end
        end

        old != value
      end