using .annotate on a queryset and output field is a result of function

[VISHESH MANGLA]

Hello,

I wanted to know that how for the model below,  how can I get a queryset with  all the fields excluding the `user` and including the  output of get_output_field? Please avoid hardcoding.

One way is

`TransactionEntry.objects.defer("users").annotate(total_price=F("num_shares")*F("price_per_share"))`

but that is undesirable. I just want to use that function on the model and not hard code the formula.

Thanks,

class TransactionEntry(models.Model):

    """Model to save data of users who want to buy/sell shares"""

    user = models.ForeignKey(

        to=Relative,

        on_delete=models.CASCADE,

        related_name="transaction_entry",

    )

    num_shares = models.PositiveIntegerField(null=False, blank=False)

    date_of_trade = models.DateField(null=False, blank=False)

    transaction_type = models.CharField(

        max_length=1,

        choices=TransactionType.choices,

        default=TransactionType.SELL,

        null=False,

    )

    price_per_share = models.FloatField(null=False, blank=False)

    def get_total_price(self):

        return self.num_shares * self.price_per_share

[Nikeet NA]

You cannot use properties in django orm , it does not allow that.
TransactionEntry.objects.defer("users").annotate(total_price=F("num_shares")*F("price_per_share")).values('total_price')

[VISHESH MANGLA]

what about overwriting the manager? How to make the manager auto-add those computed properties ?

[Nikeet NA]

You can do it like this :

class TransactionEntry(models.Manager):

       def get_total_prices(self):

             return self.queryset().defer("users").annotate.(total_price=F("num_shares")*F("price_per_share"))

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[Nikeet NA]

Its not the perfect method you can optimize ot more if you want. 

You can prefer this Link

[VISHESH MANGLA]

this looks good, thanks

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