How do you (easily) calculate total and subtotal in Django?

Question:

I'm using Django 1.7.8

I have models.py

class SaleDetail(models.Model):
    quantity = models.PositiveSmallIntegerField(_('quantidade'))
    price_sale = models.DecimalField(

    def get_subtotal(self):
        return self.price_sale * self.quantity

    subtotal = property(get_subtotal)

./manage.py shell

>>> from vendas_project.vendas.models import SaleDetail
>>> from django.db.models import Sum, F, FloatField
>>> q = SaleDetail.objects.values('price_sale', 'quantity').filter(sale=1)
>>> q.aggregate(Sum(F('price_sale') * F('quantity')), output_field=FloatField())

Generates the error:

field_list = aggregate.lookup.split(LOOKUP_SEP)
AttributeError:
    'ExpressionNode' object has no attribute 'split'

How do you calculate subtotals and total in Django?

I need the result, example:

price_sale quantity subtotal

10.50      2        21.00

9.55       3        28.65

total = 49.65

Answer:

In version 1.8 to generate the subtotal you must use annotate and to generate the total you must use aggregate .

It is necessary to use ExpressionWrapper when performing calculations with different types of values.

Example:

>>> from django.db.models import F, DecimalField, ExpressionWrapper
>>> q = SaleDetail.objects.filter(sale=1).values('price_sale', 'quantity')
>>> q_com_subtotal = q.annotate(subtotal=ExpressionWrapper(F('price_sale') * F('quantity')), output_field=DecimalField())
>>> q_com_subtotal[0].subtotal
21.0
>>> calculo_total = q_com_subtotal.aggregate(total=Sum('subtotal'))
>>> calculo_total.total
49.65

References:

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