Re: Locking / serializing access to one element in database

Hi Collin, hi all,

Am 27.10.2015 um 19:56 schrieb Collin Anderson:
> Yes, an exception will be raised.

Thinking further about this, all we need is a method that gives us an exception if we
accidentally create a second object when in fact only one is wanted. Your suggestion
with manually dealing with the PKs and using .save(force_insert=True) is one method to
achieve that, another one that works well for me is using an (application-specific)

unique_together = (year, month)

constraint, which achieves the desired result (guarantee uniqueness where required,
raise an exception otherwise) without having to manually deal with PKs.

Alas, I wonder how to proceed to complete the solution. As I find it simpler to deal
with the above mentioned unique_together rather than with coming up with a PK based
solution, I refer to my original code from [1], which was:


try:
mm = TestMonthModel.objects.select_for_update().get(jahr=Jahr, monat=Monat)
except TestMonthModel.DoesNotExist:
mm = TestMonthModel(jahr=Jahr, monat=Monat)

# A *long* computation, eventually setting fields in mm and save:

mm.value = 123
mm.save()


Combining everything from this thread, this could be changed into this code
(pseudo-code, not tested):


while True:
try:
mm = TestMonthModel.objects.select_for_update().get(jahr=Jahr, monat=Monat)
break # Got what we wanted!
except TestMonthModel.DoesNotExist:
try:
# Create the expected but missing instance.
# No matter if the following succeeds normally or throws
# an Integrity error, thereafter just restart the loop.
TestMonthModel(jahr=Jahr, monat=Monat).save()
except IntegrityError:
pass

# A *long* computation, eventually setting fields in mm and save:

mm.value = 123
mm.save()


Afaics, this solves the problem, but it also feels quite awkward and I wonder if there
is a more elegant solution.

Comments? Does this sound reasonable at all?

Best regards,
Carsten

[1] https://groups.google.com/forum/#!topic/django-users/SOX5Vjedy_s

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