Hi Carsten,
If you're just updating one field, this _might_ work for you:
try:
TestMonthModel.objects.create(jahr=Jahr, monat=Monat) # create() uses
force_insert.
except IntegrityError:
pass # It already exists. No Problem.
# ... calculate some things.
TestMonthModel.objects.filter(jahr=Jahr, monat=Monat).update(value=new_value
)
Collin
On Wednesday, October 28, 2015 at 8:50:09 PM UTC+1, Carsten Fuchs wrote:
>
> 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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