Right now, it is not possible to sort via user rating… nor is it possible to access the user rating via the Agent Plug-in API (meaning I can’t write an agent that writes the user rating to the sortable rating field).
Please either make it possible to sort via user rating, or expose that field to third-party developers so that they may utilize it. Right now, to work around this I’m using external tools to watch for changes to the Library database, and copy user ratings between tables. It’s a very clunky and unwieldy way to solve what should be a simple problem.
