How to store session values with Node.js and mongodb?
Take a look at this series from DailyJS. It uses MongoDB and session management
I am not experienced with Node.js or Express, so I cannot immediately see what's wrong with your approach. However, I have made Express use MongoDB to store sessions for flash messages and other session stuff.
You can see my source code for a simple URL shortener here (that actually makes the URLs pretty long at the moment - it was just an exercise ;)). I use the session to store a list of URLs that the current user has shortened.
It is not pretty, but I know it works.