instance method field_changed?

# File activerecord/lib/active_record/attribute_methods/dirty.rb, line 74
      def field_changed?(attr, old, value)
        if column = column_for_attribute(attr)
          if column.number? && column.null && (old.nil? || old == 0) && value.blank?
            # For nullable numeric 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 ''.
            # If an old value of 0 is set to '' we want this to get changed to nil as otherwise it'll
            # be typecast back to 0 (''.to_i => 0)
            value = nil
          else
            value = column.type_cast(value)
          end
        end

        old != value
      end