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) 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?
Use an F expression:
from django.db.models import F
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.