public void Rate(long id, int rating)
{
Book book = dao.FindById(id);
book.Add(new SingleRating(rating));
dao.Save(book);
}
public void Borrow(long userId, long bookId)
{
var user = userDao.FindById(userId);
var book = bookDao.FindById(bookId);
borrowingDao.Insert(new Borrowing(user, book));
}