Question

Tricky code:

user = User.objects.filter(id=123)
user[0].last_name = 'foo'
user[0].save()  # Cannot be saved.
id(user[0])     # 32131
id(user[0])     # 44232 ( different )

user cannot be saved in this way.

Normal code:

user = User.objects.filter(id=123)
if user:
  user[0].last_name = 'foo'
  user[0].save()  # Saved successfully.
  id(user[0])     # 32131
  id(user[0])     # 32131 ( same )

So, what is the problem?

Answer 1

In first variant your user queryset isn’t evaluated yet. So every time you write user[0] ORM makes independent query to DB. In second variation queryset is evalutaed and acts like normal Python list.

And BTW if you want just one row, use get:

user = User.objects.get(id=123)