I’m working on a project utilizing notifications as well, I’m not sure if you got yours sorted out by now or if it might help but this is the table structure I used:
Notifications:
- ID (PK)
- recipient_id
- sender_id
- activity_type ('comment on a post', 'sent friend request', etc)
- object_type ('post', 'photo', etc)
- object_url (to provide a direct link to the object of the notification in HTML)
- time_sent
- is_unread