text: Title: db.session.add(..) but without commit(()
  Body: Hi, i'm reading your book and at this point in Chapter 10. In model.py the "User" class has several db.session.add(self) but no db.session.commit(). I thought that every time an update in database there should be an commit() in the end but your code works fine. So i'm a little confused. Thank you for your patient.
  