Django select_for_update cannot be used outside of a transaction

Question

I was using Django 1.5.1 and upgraded to Django 1.6.6.

In Django 1.5.1 I was using select for update to guarantee atomic execution.

job_qs = Job.objects.select_for_update().filter(pk=job.id)
for job in job_qs:

Unfortunately this now throws an error:

  File "/srv/venvs/django-picdoc/local/lib/python2.7/site-packages/django/db/models/query.py", line 96, in __iter__
    self._fetch_all()

  File "/srv/venvs/django-picdoc/local/lib/python2.7/site-packages/django/db/models/query.py", line 857, in _fetch_all
    self._result_cache = list(self.iterator())

  File "/srv/venvs/django-picdoc/local/lib/python2.7/site-packages/django/db/models/query.py", line 220, in iterator
    for row in compiler.results_iter():

  File "/srv/venvs/django-picdoc/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 713, in results_iter
    for rows in self.execute_sql(MULTI):

  File "/srv/venvs/django-picdoc/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 776, in execute_sql
    sql, params = self.as_sql()

  File "/srv/venvs/django-picdoc/local/lib/python2.7/site-packages/django/db/models/sql/compiler.py", line 147, in as_sql
    raise TransactionManagementError("select_for_update cannot be used outside of a transaction.")

TransactionManagementError: select_for_update cannot be used outside of a transaction.

What are some of the solutions for solving this?

Answer 1

The answer is in the error, wrap the query in a transaction

Django's documentation is located here: https://docs.djangoproject.com/en/dev/topics/db/transactions/#django.db.transaction.atomic

One approach is:

from django.db import transaction

def some_method():    
   with transaction.atomic():
      job_qs = Job.objects.select_for_update().filter(pk=job.id)
      for job in job_qs:

Answer 2

Addendum

As of Django 2.0, related rows are locked by default (not sure what the behaviour was before) and the rows to lock can be specified in the same style as select_related using the of parameter:

By default, select_for_update() locks all rows that are selected by the query. For example, rows of related objects specified in select_related() are locked in addition to rows of the queryset’s model. If this isn’t desired, specify the related objects you want to lock in select_for_update(of=(...)) using the same fields syntax as select_related(). Use the value 'self' to refer to the queryset’s model.

https://docs.djangoproject.com/en/dev/ref/models/querysets/#select-for-update