On facebook, for instance, when you log in you have a number of unread notifications. Once you click on your notifications, those become "read" and the number disappear. In the notification list, the notifications that you actually clicked changes background color, so even tho you don't have an unread notification count, you can still see which notifications you actually visited. Maybe having two flags is not the solution. We could have an `unread_notification_count` stored on the `activity_object`, and a simple `read` flag on the notification itself.