update django models column based on other column - django

Question

I have a model like
class tbl_payment(models.Model):
document_id = models.ForeignKey(tbl_invoice, on_delete=models.CASCADE)
client_id = models.ForeignKey(tbl_customer, on_delete=models.CASCADE)
total_amount = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
paid_amount = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
balance = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
date = models.DateField(blank=True, null=True)
status = models.CharField(max_length=50)
Now what I want to do is whenever a new record is added, or an existing record changes, balance should be updated as the difference between total_amount and paid_amount (simple maths), and based on my balance column, I want to save status as Paid, Partial or Unpaid.
I want to refrain from calculating the balance in my views and then saving in the database, instead, I want to handover this part to my models so that my models take care of the balance and I may avoid errors which I am subconsciously afraid of.
I came to know that this is done something like this
class tbl_payment(models.Model):
document_id = models.ForeignKey(tbl_invoice, on_delete=models.CASCADE)
client_id = models.ForeignKey(tbl_customer, on_delete=models.CASCADE)
total_amount = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
paid_amount = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
balance = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
date = models.DateField(blank=True, null=True)
status = models.CharField(max_length=50)
#property
def balance(self, value):
return self.total_amount - paid_amount
but what should I pass in place of value??
also when I try to get due_amount value like this
tbl_bulk_payment.objects.get(pk=1).due_amount
it gives due_amount() missing 1 required positional argument: 'value'
what is the correct way of doing this??

Answer 1

you have to override save() function
class tbl_payment(models.Model):
class Status(models.TextChoices):
paid = 'Paid', 'Paid'
partial = 'Partial', 'Partial'
unpaid = 'Unpaid', 'Unpaid'
document_id = models.ForeignKey(tbl_invoice, on_delete=models.CASCADE)
client_id = models.ForeignKey(tbl_customer, on_delete=models.CASCADE)
total_amount = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
paid_amount = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
balance = models.DecimalField(max_digits=8, decimal_places=2, blank=True, null=True)
date = models.DateField(blank=True, null=True)
status = models.CharField(max_length=50, choices=Status.choices)
def save(self, *args, **kwargs):
self.balance = self.total_amount - self.paid_amount
if self.balance >= 0:
self.status = Status.paid
elif self.paid_amount == 0:
self.status = Status.unpaid
else:
self.status = Status.partial
super(tbl_payment, self).save(*args, **kwargs)
P.S. your Model class name is not following the python or Django naming rule.
class Payment(models.Model) would be better.
and document_id => document. because
payment = Payment.objects.get(pk=1)
payment.document_id
in this case payment.document_id would return document instance. (not document instance's id). so document is more Django like style than document_id