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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