Agfdhyk

Good evening.
The goal is that when creating the Transfer model, the sum field was subtracted from whom_from and added to whom_to Bill's model. As I understand it, this needs to be done through save (), but apparently, I did not set it up correctly. What could be the problem?

models.py

class Bill(models.Model):

    title = models.CharField(max_length=100)

    amount = models.DecimalField(max_digits=19, decimal_places=2, 

                                 validators=[MinValueValidator(0)], default=0)



    def __str__(self):

        return self.title





class Transfer(models.Model):

    whom_from = models.ForeignKey(Bill, on_delete=models.CASCADE, related_name='whom_from')

    whom_to = models.ForeignKey(Bill, on_delete=models.CASCADE, related_name='whom_to')

    sum = models.DecimalField(max_digits=19, decimal_places=2, validators=[MinValueValidator(0)], default=0)



    def save(self, *args, **kwargs):

        self.whom_to.amount = self.whom_to.amount + self.sum

        self.whom_from.amount = self.whom_from.amount - self.sum

        super(Transfer, self).save(*args, **kwargs)