Django - Update model field based on another field

Question

I am new to Django and Python and I want to do something I used to do very often in Java EE.

Consider the following model (only relevant classes):

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Answer 1

See if this works for you. We override the save method and check if there is a pk. If the pk is None, we know that the SaleDetail is new. Then, we see if there is a unit_price. If there is not a unit_price, we set it to the Item.default_price.

class Item(models.Model):
    name = models.CharField(max_length=40)
    default_price = models.DecimalField(max_digits=6, decimal_places=2, default=50)

    def __unicode__(self):
        return self.name


class SaleDetail(models.Model):
    item = models.ForeignKey(Item)
    deposit = models.ForeignKey(Deposit)
    quantity = models.PositiveIntegerField()
    unit_price = models.DecimalField(max_digits=6, decimal_places=2)
    sale = models.ForeignKey(Sale)

    def save(self, *args, **kwargs):
        # Make sure this is the first save (pk should be None) and there is no unit_price set
        if self.pk is None and not self.unit_price:
            self.unit_price = self.item.default_price
        elif not self.unit_price:
            self.unit_price = self.item.default_price

        # Call the original save method
        super(SaleDetail, self).save(*args, **kwargs)

Answer 2

The "unit price" is a literally a function of two different fields. As such I would tend to write it like this:

class Item(models.Model):
    name = models.CharField(max_length=40)
    default_price = models.DecimalField(max_digits=6, decimal_places=2, default=50)

    def __unicode__(self):
        return self.name

class SaleDetail(models.Model):
    item = models.ForeignKey(Item)
    deposit = models.ForeignKey(Deposit)
    quantity = models.PositiveIntegerField()
    entered_unit_price = models.DecimalField(max_digits=6, decimal_places=2, default=None)
    sale = models.ForeignKey(Sale)

    @property
    def unit_price(self, value):
        if self.entered_unit_price is None:
            return self.item.default_price
        else:
            return self.entered_unit_price

    @unit_price.setter
    def unit_price(self, value):
        self.entered_unit_price = value

Then use it like so:

print(sd.unit_price)
sd.unit_price = 500 
sd.save()