Ordering by timestamp on optional field

[Mike Gaffney <mr...@gmail.com>]

I have a rating system where users rate items. the item is the row
key, the user is the column key. The value is the rating on a 5 point
scale. People can only have one rating per item.

I'm looking to add the ability to optionally add a text comment to the
rating record. If the text comment is not null or empty I'd like to be
able to find the rating by item in timed order.  If the person updates
their comment I'd like to update the time in the index.

I'm using Cassandra 0.7.5.

Any suggestions?

Thanks,
  -=gaffo=-