I think you, ww520, have a very well point here and I'm also interesten in what RethinkDB can offer for this very usage scenario. From what I read from the ReQL command reference there it should be possible to do something like:

  r.table('foo').get(5).update({ 'bar': r.branch(r['baz'] == 0, "foo", r.error("invalid baz!"))})

have not tested it, but this is how I understand it...