field_changed?

# File activerecord/lib/active_record/dirty.rb, line 143
      def field_changed?(attr, old, value)
        if column = column_for_attribute(attr)
          if column.type == :integer && column.null && old.nil?
            # For nullable integer columns, NULL gets stored in database for blank (i.e. '') values.
            # Hence we don't record it as a change if the value changes from nil to ''.
            value = nil if value.blank?
          else
            value = column.type_cast(value)
          end
        end

        old != value
      end