Rails: Keeping count of users unread notifications

I currently have an activity model that handles a user activity notification system. An activity observer creates a new activity when some action happens (such as a new article being created). Now I want to keep a record of how many of these activity notifications the current user has not seen (similar to the notification jewel on facebook). Every time the user clicks on their notifications link the number should reset to 0 and every notification created should increase the count by 1. Where would I store this data for each user? Would using a session work or is something else better?