On Wed, Sep 16, 2009 at 7:32 PM, Russell Keith-Magee
<freakboy3...@gmail.com> wrote:
> On Wed, Sep 16, 2009 at 9:36 AM, Brent Hagany <brent.hag...@gmail.com> wrote:
>>
>> I'm having some trouble getting F() expressions to obey my parentheses
>> when I don't want the default order of operations. For example, given
>> the model:
>>
>> class MyModel(models.Model):
>> wins = models.DecimalField(max_digits=1, decimal_places=0)
>> losses = models.DecimalField(max_digits=1, decimal_places=0)
>> win_percentage = models.DecimalField(max_digits=4,
>> decimal_places=3, default=Decimal('0.000'))
>>
>> I get the following results when trying to calculate the
>> win_percentage:
>>
>> In [1]: MyModel.objects.create(wins=2, losses=4)
>> Out[1]: <MyModel: MyModel object>
>>
>> In [2]: MyModel.objects.all().update(win_percentage=F('wins') / (F
>> ('wins') + F('losses')))
>> Out[2]: 1
>>
>> # I expect this to return Decimal("0.333")
>> In [3]: MyModel.objects.get(pk=1).win_percentage
>> Out[3]: Decimal("5.000")
>>
>> It appears to be ignoring the parentheses around F('wins') + F
>> ('losses'), and so instead of 2 / (2 + 4) = .333, I'm getting 2 / 2 +
>> 4 = 5. Am I doing this wrong, or is this by design, or is it a bug?
>
> This appears to be a bug - and a fairly embarrassing one at that.
> Although the parentheses are correctly parsed, the query evaluator
> doesn't appear to be using that detail when outputting the final
> query.
>
> I've opened ticket #11886 to track this problem. Looking at the code,
> it seems to be caused by a fairly dumb mistake in the query evaluator;
> if I'm correct, I should be able to commit a fix this evening.
FYI: I've fixed this bug in trunk r11581; the 1.1.X branch has the
same fix as r11582.
Yours,
Russ Magee %-)
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