Patch for caching columns for has_and_belongs_to_many

I submitted a patch to lighthouse which caches columns ("SHOW FIELDS")
for has_and_belongs_to_many associations. See the patch and ticket
at: http://rails.lighthouseapp.com/projects/8994/tickets/1738-caching-columns-for-has_and_belongs_to_many

The patch is simple and unobtrusive, and resulted in a 25% performance
increase (response time, CPU load, and database load all dropped by
about 25%) in my companies Rails app (http://teamsporttech.com) which
serves over 500 websites for sports teams.

Further details are at
http://lukeludwig.com/blog/2009/01/08/rails-patch-for-caching-show-fields-for-has_and_belongs_to_many-associations

If someone could take a look at this patch that would be great,

Luke