#25271: model object locking with select_related
-------------------------------------+-------------------------------------
Reporter: VathsalaAchar | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: 1.8
(models, ORM) | Keywords: models, select_related,
Severity: Normal | resource locking
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
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I used select_related (django 1.6 on windows connected to sqlserver) and
came across some unexpected behaviour.
Here's an example that I tested with django 1.8 and sqlite:
{{{
class TestUser(models.Model):
name = models.CharField(blank=True, max_length=255)
class Account(models.Model):
user = models.OneToOneField(TestUser)
name = models.CharField(max_length=255)
value = models.IntegerField(default=0)
}}}
I tried the following code against the models above to replicate the
unexpected behaviour I noticed.
{{{
def some_method(some_id):
t = TestUser.objects.select_related('account').get(id=some_id)
# here is where the fun stuff happens
alter_account(t.id)
# the following changes are reflected in the db
t.account.name= 'something else'
t.account.save()
def alter_account(uid):
'''
this method does not work as expected as the select_related in the
previous method has locked the Account model out
'''
a = Account.objects.get(user__id=uid)
a.value = 5
a.save()
test_user = TestUser.objects.create(name='test user')
test_account = Account.objects.create(user=test_user, name='some account')
}}}
Initial state:
{{{
>>> test_account.name
u'some account'
>>> test_account.value
0
}}}
Expected:
{{{
>>> test_account.name
u'something else'
>>> test_account.value
5
}}}
Unexpected result:
{{{
>>> some_method(test_user.id)
>>> test_account.name
u'something else'
>>> test_account.value
0
}}}
I found this in our production code when the object state refused to
update and caused us issues. I had to get rid of the select_related in the
query to resolve the issue.
Nowhere in the documentation is this behaviour explained, so I assume this
is a bug.
--
Ticket URL: <https://code.djangoproject.com/ticket/25271>
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