Calculated Fields

[tech george]

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,

[Mike Dewhirst]

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If it was my code, I would extract the calculation into a model method and call it from the save method as you are doing.

That would make it easier to unit-test and perhaps discover that there should be parens around the addition.

You also need to call save on your model to execute the code.

Apart from that, at first glance your code should work.

Regards,

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[tech george]

Hello Mike,

Unfortunately it's not calculating the fields as I wanted, instead it's calculating as below;

 

reorder_level * unit_price + quantity

Which is wrong because I'm getting less amount.

Regards

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[Mike Dewhirst]

On 11/04/2022 2:48 pm, tech george wrote:

Hello Mike,

Unfortunately it's not calculating the fields as I wanted, instead it's calculating as below;

 

reorder_level * unit_price + quantity

Try using brackets like this ...

self.total_price = (self.reorder_level + self.quantity) * self.unit_price

Although I might use an error preventer. Perhaps like this ...

if self.unit_price and (self.reorder_level or self.quantity):
    self.total_price = (self.reorder_level + self.quantity) * self.unit_price

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[Antonis Christofides]

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

[tech george]

Hello Antonis,

Thanks for the advice and the solution,

That solved my issue.

Thanks again.

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[Jet Ezra]

I think you can use the legendary BODMAS, and apply brackets

self.total_price = self.quantity+(self.reorder_level * self.unit_price)

That should work

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--

jet

[Ahmedrufai Otuoze]

Hi Mike, 

I can help you solve the problems.

I can clone it from your repo and we'll work on it...

What do you think?

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[harsh jain]

Hi,

This course will help you surely - https://geekster.in/

[Kasper Laudrup]

On 13/04/2022 07.20, harsh jain wrote:
> Hi,
> This course will help you surely - https://geekster.in/
>

Stop spamming. This is *not* the way to attract potential clients.

I would never ever consider using your company for anything and I hope
others feel the same.

[Ahmedrufai Otuoze]

This is not a spam. I just tok a course on that aspect and wanted to help. 

No strings attached. 

I may have gone against the policy and that's because I wasn't aware.

My intentions were genuine...

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[Kasper Laudrup]

On 13/04/2022 16.51, Ahmedrufai Otuoze wrote:
> This is not a spam. I just tok a course on that aspect and wanted to help.
> No strings attached.
>
> I may have gone against the policy and that's because I wasn't aware.
> My intentions were genuine...
>

You were not the one I was replying to so I don't understand why you are
replying like you were advertising this.

Harsh Jain has previously been spamming this mailing list and he's the
one I was replying to.

Kind regards,

Kasper Laudrup

[Ahmedrufai Otuoze]

My bad. I bypassed your message completely.

Have a great day!

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