Question

I seem to be having a problem multiplying in Django. I want to multiply the Total Purchase including VAT with my VAT value. Unfortunately, I got this error.

unsupported operand type(s) for *: 'Decimal' and 'VAT'

It looks like my program calls vat (see views) `’VAT’ rather than decimal. I’m not sure why there is an error unless there is another way around it.

Views.py

purchases = Purchase.objects.all()
vat = purchases[0].vat
total_purchases = Purchase.objects.all().aggregate(price = Sum('amount'))
total_purchases_vat = total_purchases['price'] * vat

models.py

class VAT(models.Model):
    vat = models.DecimalField(max_digits = 4, decimal_places = 1)
    def __unicode__(self):
        return unicode(self.vat)

class Purchase(models.Model):
    vat_period = models.ForeignKey(VAT_Period)
    date = models.DateField()
    amount = models.DecimalField(max_digits=20, decimal_places=2)
    description =  models.TextField(max_length = 400)
    vat = models.ForeignKey(VAT)
    def __unicode__(self):
        return unicode(self.amount)

Answer 1

purchases[0].vat yields an instance of the VAT model. You need to use purchases[0].vat.vat to get the actual value.