Atomic increment of a counter in django

Question

I'm trying to atomically increment a simple counter in Django. My code looks like this:

from models import Counter from django.db import transaction  @transaction.commit_on_success def increment_counter(name):     counter = Counter.objects.get_or_create(name = name)[0]     counter.count += 1     counter.save() 

If I understand Django correctly, this should wrap the function in a transaction and make the increment atomic. But it doesn't work and there is a race condition in the counter update. How can this code be made thread-safe?

Answer 1

In Django 1.4 there is support for SELECT ... FOR UPDATE clauses, using database locks to make sure no data is accesses concurrently by mistake.

Answer 2

Use an F expression:

from django.db.models import F 

either in update():

Counter.objects.get_or_create(name=name) Counter.objects.filter(name=name).update(count=F("count") + 1) 

or on the object instance:

counter, _ = Counter.objects.get_or_create(name=name) counter.count = F("count") + 1 counter.save(update_fields=["count"]) 

Remember to specify update_fields, or you might encounter race conditions on other fields of the model.

A note on the race condition avoided by using F expressions has been added to the official documentation.