Better way to prevent AR from updating certain columns?

Hi,
I've been helping Jean-Christophe Michel with the Better Nested Set
plugin (http://opensource.symetrie.com/trac/better_nested_set), and we
need a way to make ActiveRecord exclude certain columns from the UPDATE
statement. My solution so far is to override the update method in AR,
forcing it to ignore the columns in question. Is this a reasonable way
to do things? It seems sort of heavy-handed. I've included the code in
question below.

The situation is a little hard to explain without discussing how a
nested set works, but I'll try: there are two index columns which
define a tree structure, and whose values are updated by direct SQL
statements only, and never through AR. No big deal, you say: just
override the setter methods for those attributes. And that is exactly
what we do. But the problem is this: the index columns get updated for
large portions of the table any time the tree structure is altered, so
the index values of an AR object can easily become stale. If the record
is then saved, the stale index values are written to the database,
corrupting the index.

Any input is appreciated.

Krishna

          # override ActiveRecord's 'update' to make update call my
'special_attributes_with_quotes'
          def update #:nodoc:
            connection.update(
              "UPDATE #{self.class.table_name} " +
              "SET #{quoted_comma_pair_list(connection,
special_attributes_with_quotes(false))} " +
              "WHERE #{self.class.primary_key} = #{quote_value(id)}",
              "#{self.class.name} Update"
            )
          end

          # exclude the lft/rgt columns from update statements
          def special_attributes_with_quotes(include_primary_key =
true) #:nodoc:
            attributes.inject({}) do |quoted, (name, value)|
              if column = column_for_attribute(name)
                quoted[name] = quote_value(value, column) unless
(!include_primary_key && column.primary) ||
[acts_as_nested_set_options[:left_column],
acts_as_nested_set_options[:right_column]].include?(column.name)
              end
              quoted
            end
          end

          # i couldn't figure out how to call attributes_with_quotes
without cutting and
          # pasting this private method in.
          # Quote strings appropriately for SQL statements.
          def quote_value(value, column = nil) #:nodoc:
            self.class.connection.quote(value, column)
          end