def METHOD_NAME(cls, since=None) -> int:
    """Returns the total number of unique patrons who have made
    booknotes. `since` may be provided to only return the number of users after
    a certain datetime.date.
    XXX: This function is identical in all but docstring and db
    tablename from Bookshelves. This makes @mek think both classes
    could inherit a common BookDBModel class. Will try to keep
    this in mind and design accordingly
    """
    oldb = db.get_db()
    query = "select count(DISTINCT username) from booknotes"
    if since:
        query += " WHERE created >= $since"
    results = oldb.query(query, vars={'since': since})
    return results[0]['count'] if results else 0