On 05/18/2010 03:43 PM, Ian Lewis wrote:
> Roland,
>
> Instead of using annotate, could you just add the avg_days_instock
> field as another key in the select argument to extra?
>
Well how would I do that?
Since I want the avg_days_instock calculated on a 'group_by' basis.
To me it seems that I need to do a Count and a avg_days_instock at the
same time?
Roland
> Ian
>
> On Tue, May 18, 2010 at 9:03 PM, Roland van Laar <rol...@micite.net> wrote:
>
>> Hello,
>>
>> Question: How do I calculate the Average of a calculated field.
>>
>> My (simplified) models.py:
>>
>> Manufacturer(models.Model):
>> name = models.CharField(length=100)
>>
>> Product(models.Model):
>> name = models.CharField(length=100)
>>
>> Items(models.Model):
>> manufacturer = models.Foreignkey(Manufacturer)
>> product = models.ForeignKey(Product)
>> bought_date = models.DateField()
>> sold_date = models.DateField(null=True, blank=True)
>>
>> On these models I want to calculate the number of days an item was in stock
>> on a given date:
>>
>> qs = Items.objects.all().extra(select= \
>> {'days_in_stock': "'%s' - transactie_datum_inkomend + 1" %
>> (given_date.isoformat())})
>>
>> And then I want to know the average number of days all the items selected on
>> manufacturer and product.
>>
>> qs.values('manufacturer', 'product)
>> qs.annotate(Count('manufacturer'), avg_days_instock=Avg('days_in_stock'))
>>
>> The annotate fails because 'days_in_stock' is not a valid field:
>> FieldError: Cannot resolve keyword 'days_in_stock' into field. Choices are:
>> manufacturer, product, bought_date, sold_date
>>
>> How can I calculate the average of the days_in_stock?
>>
>> Thanks,
>>
>> Roland van Laar
>>
>>
>>
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>
>
>
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