CODEWITHSUNDEEP
pythondjangodjango-models
Paulius Stundžia
Paulius Stundžia

Reputation: 332

Django property field calculated from other property fields

Does anyone know a way in Django to calculate a field from the values of other property fields? Here's my invoice model:

class Invoice(models.Model):
    types = [
        ('customer', 'Customer'),
        ('supplier', 'Supplier')
    ]

    number = models.CharField(max_length=30)
    date_invoice = models.DateTimeField('Date', default=timezone.now())
    comment = models.TextField(default=get_default_invoice_comment())
    type = models.CharField(max_length=20, choices=types, default='customer')
    partner_id = models.ForeignKey(
        'Partner', default=1, on_delete=models.CASCADE)
    currency_id = models.ForeignKey(
        'Currency', default=1, on_delete=models.CASCADE)

    @property
    def tax_amount(self):
        total = 0
        for line in self.invoiceline_set.all():
            total += line.tax_amount
        return total

    @property
    def subtotal(self):
        total = 0.0
        for line in self.invoiceline_set.all():
            total += line.subtotal
        return total

    @property
    def total(self):
        return self.subtotal + self.tax_amount

    def __unicode__(self):
        return self.number

The tax_amount and subtotal on the InvoiceLine model are property fields. The subtotal gets calculated fine, however the total does not as it depends on another property field (tax_amount, which also does not get calculated properly) and I guess Django does not know in what order to perform the calculations? To reiterate, the problem is the total and tax_amount fields are always empty (even if the fields they are calculated from are not).

Upvotes: 0

Views: 1203

Answers (1)

Paulius Stundžia
Paulius Stundžia

Reputation: 332

The problem was that some invoice_lines did not have a tax_id (because they were added before that was implemented) and the tax_amount field on the InvoiceLine is a property field that is calculated by the rate on the related Tax object. So the tax_amount on those lines wasn't able to be computed as it tried to sum floats and None types.

Upvotes: 1

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