User subscription to a discussion. When a user subscribes to a discussion he/she will receive emails of any posts in this discussion.
| Column | Type | References | Comments |
| id | Integer | Primary key | |
| vdb_id | Integer | ||
| created | Date | ||
| discussion_id | Integer | db_discussion.id | Topic subscribed to |
| user_id | Integer | db_person.id | ID of the subscriber |