That was my idea too, but using this way, the pre_save / post_save signals
wont be triggered.
It seems strange to me to be using the objects manager instead of the model
instance directly, no?
Seb
Le mardi 21 août 2012 02:11:42 UTC+2, Thomas Orozco a écrit :
>
> As a followup to the suggestion of MyModel.objects.filter(money__gte =
> value, pk = self.pk).update(F...)
>
> Here's an example:
>
> We have testapp/models.py:
>
> from django.db import models
> class TestModel(models.Model):
> balance = models.IntegerField()
>
>
> >>> from django.db.models import F
> >>> TestModel.objects.create(balance = 5) #Pk will be 1 I just create one.
> >>> import logging
> >>> l = logging.getLogger('django.db.backends')
> >>> l.setLevel(logging.DEBUG)
> >>> l.addHandler(logging.StreamHandler())
> >>> TestModel.objects.filter(balance__gte = 4, pk = 1).update(balance =
> F('balance') - 4)
> (0.001) UPDATE "testapp_testmodel" SET "balance" =
> "testapp_testmodel"."balance" - 4 WHERE ("testapp_testmodel"."balance" >= 4
> AND "testapp_testmodel"."id" = 1 ); args=(4, 4, 1)
> 1
> >>> TestModel.objects.filter(balance__gte = 4, pk = 1).update(balance =
> F('balance') - 4)
> (0.000) UPDATE "testapp_testmodel" SET "balance" =
> "testapp_testmodel"."balance" - 4 WHERE ("testapp_testmodel"."balance" >= 4
> AND "testapp_testmodel"."id" = 1 ); args=(4, 4, 1)
> 0
>
>
>
> *So this seems to generate a single SQL statement.*
> *
> *
> *I'm not totally familiar with database administration though, so as** Melvyn
> rightly pointed out, it's always better if you can get the extra security
> of having an SQL constraint into your dabatase and wrap your queries in a
> transaction (as you'll probably be adding a line to the statement if the
> withdrawal succeeds).*
> *
> *
>
> 2012/8/20 Thomas Orozco <g.orozc...@gmail.com >
>
>> A few suggestions :
>>
>> Circumvent the problem with smarter design: don't store the money on the
>> object, make the user's money the sum of all their transactions (credit -
>> debit).
>> You get lesser performance, but you also get history!
>>
>> Maybe you could try (not sure about that):
>>
>> MyModel.objects.filter(money__gte = value, pk = self.pk).update(F...)
>>
>> and inspect the return value (number of updated rows!).
>> Now, you'd need to make sure django does that in a single statement.
>>
>> If that doesn't work, I think update is the way to go anyway, but it
>> might get a bit messy.
>>
>> F... is an F object whose syntax I don't have off the top of my head.
>> Le 20 août 2012 18:54, "Sebastien Flory" <sfl...@gmail.com >
>> a écrit :
>>
>> Hi everyone,
>>>
>>> I'm looking for the proper django way to do an update of an attribute on
>>> my model instance, but only if the attribute current value is checked
>>> agains't a condition, in an atomic way, something like this:
>>>
>>> def use_money(self, value):
>>> begin_transaction()
>>> real_money = F('money')
>>> if real_money >= value:
>>> self.money = F('money') - value
>>> self.save()
>>> end_transaction()
>>>
>>> I want to make sure that I avoid race condition so money never goes
>>> below 0.
>>>
>>> Can you help me out?
>>>
>>> Thanks,
>>>
>>> Sebastien
>>>
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>
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