Re: Calculated Fields

As was found out with the conversation with Mike Dewhirst, the error appears to be because of wrong understanding of operator precedence. That's easy to correct with parentheses.

Except for that, I wouldn't do it that way; I wouldn't store the total price in the database, because it's likely to create problems. Models that have not (yet) been saved to the database will not have a total price. Models that have been loaded from the database, modified, but not (yet) updated in the database will have a wrong total price. Saving to the database with bulk operations or with plain SQL may result in no total price. These are all symptoms of storing redundant information in the database, which is something that should be avoided.

Instead, I would do this:

class Stock(models.Model):
    unit_price = models.DecimalField(max_digits=10, decimal_places=2, default='0', blank=True, null=True)
    quantity = models.IntegerField(default='0', blank=True, null=True)
    reorder_level = models.IntegerField(default='0', blank=True, null=True)

    @property

    def total_price(self):

        return (self.quantity + self.reorder_level) * self.unit_price

On 10/04/2022 20.49, tech george wrote:

Hello,

I am trying to calculate fields directly from a model but no luck. I want to get total_price from quantity, reoder_level and unit price. My code is abelow, Please advise.

class Stock(models.Model):
    unit_price = models.DecimalField(max_digits=10, decimal_places=2, default='0', blank=True, null=True)
    quantity = models.IntegerField(default='0', blank=True, null=True)
    total_price = models.DecimalField(max_digits=10, decimal_places=2,default=1)
    reorder_level = models.IntegerField(default='0', blank=True, null=True)

    def save(self, *args, **kwargs):
        self.total_price = self.quantity + self.reorder_level * self.unit_price
        super(Stock, self).save(*args, **kwargs)

What am i my doing wrong

Regards,

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