Question

I have a simple django’s query set like:

qs = AModel.objects.exclude(state="F").order_by("order")

I’d like to use it as follows:

qs[0:3].update(state='F')
expected = qs[3] # throws error here

But last statement throws:
“Cannot update a query once a slice has been taken.”

How can I duplicate the query set?

Answer 1

It’s the first line throwing the error: you can’t do qs[0:3].update(). qs[0:3] is taking a slice; update() is updating the query.

update() is meant for bulk updates, resulting in SQL queries like

UPDATE app_model SET state = 'F' WHERE state <> 'F';

You’re trying to update the first three items according to “order”, but that can’t be done with this type of UPDATE–you can’t order or limit an SQL UPDATE. It needs to be written differently, eg.

UPDATE app_model SET state = 'F' WHERE id IN (
    SELECT id FROM app_model WHERE state <> 'F' ORDER BY order LIMIT 3
) AS sub;

but Django can’t do that for you.