Comment by scottmessinger

It appears as if rank is stored as a float. The rank can quickly get to be a very, very small number. It seems at some point, float precision might be an issue and/or length of the rank key. Have you run into this and/or do you have solutions?

Brandon Keepers wrote about trying this idea in Ruby http://opensoul.org/blog/archives/2011/09/06/sortable-lists-... and ran into a float precision issue (see comment).