How should one handle a possible race condition in a model's save()
method?
For example, the following example implements a model with an ordered list of related items. When creating a new Item the current list size is used as its position.
From what I can tell, this can go wrong if multiple Items are created concurrently.
class OrderedList(models.Model):
# ....
@property
def item_count(self):
return self.item_set.count()
class Item(models.Model):
# ...
name = models.CharField(max_length=100)
parent = models.ForeignKey(OrderedList)
position = models.IntegerField()
class Meta:
unique_together = (('parent','position'), ('parent', 'name'))
def save(self, *args, **kwargs):
if not self.id:
# use item count as next position number
self.position = parent.item_count
super(Item, self).save(*args, **kwargs)
I've come across @transactions.commit_on_success()
but that seems to apply only to views. Even if it did apply to model methods, I still wouldn't know how to properly handle a failed transaction.
I am currenly handling it like so, but it feels more like a hack than a solution
def save(self, *args, **kwargs):
while not self.id:
try:
self.position = self.parent.item_count
super(Item, self).save(*args, **kwargs)
except IntegrityError:
# chill out, then try again
time.sleep(0.5)
Any suggestions?
Update:
Another problem with the above solution is that the while
loop will never end if IntegrityError
is caused by a name
conflict (or any other unique field for that matter).
For the record, here's what I have so far which seems to do what I need:
def save(self, *args, **kwargs):
# for object update, do the usual save
if self.id:
super(Step, self).save(*args, **kwargs)
return
# for object creation, assign a unique position
while not self.id:
try:
self.position = self.parent.item_count
super(Step, self).save(*args, **kwargs)
except IntegrityError:
try:
rival = self.parent.item_set.get(position=self.position)
except ObjectDoesNotExist: # not a conflict on "position"
raise IntegrityError
else:
sleep(random.uniform(0.5, 1)) # chill out, then try again
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