SAWarning: Usage of the 'Session.add()' operation is not currently supported within the execution stage

I'm trying to do some operations after an insert occurs on a particular table.

user = ModelFactory.create_user(username, email, password)
db.session.add(user)
db.session.commit()

So I have created a method which invokes automatically on after_insert.

def notify_user(user):
    print user.id
    book = Book(author=user.id, name='foo')
    db.session.add(book)
    db.session.commit(book)

@event.listens_for(User, 'after_insert')
def receive_after_insert(mapper, connection, target):
    print target
    notify_user(target)

But this code shows a warning like,

SAWarning: Usage of the 'Session.add()' operation is not currently supported within the execution stage of the flush process. Results may not be consistent.  Consider using alternative event listeners or connection-level operations instead.
  % method)

/home/avinash/.virtualenvs/s2s/local/lib/python2.7/site-packages/sqlalchemy/util/langhelpers.py:68: SAWarning: An exception has occurred during handling of a previous exception.  The previous exception is:
 <class 'sqlalchemy.exc.ResourceClosedError'> This transaction is closed

This post shows that we have to do the work in the before_flush. I tried moving the logic inside before_flush method but it shows that the user.id is None. Yes, that's expected since the entity won't get committed to db.

Likewise I have tried after_flush event but still I get the same warning.

after_flush event does work:

@event.listens_for(User, "after_insert")
def receive_after_insert(mapper, connection, user):
    print(user.id)
    @event.listens_for(Session, "after_flush", once=True)
    def receive_after_flush(session, context):
        session.add(Book(author=user.id, name="foo"))

But why wouldn't you want to make User.author a relationship instead?