Django: filter a query with an object attribute

Question

I'm trying to build a filter depending on if an object attribute is empty or not.

Below my 2 class tables:

Buy model
id   buy_price sell_price
10   15        50
20   30        110
30   80        250

Sell model
item buyer*
10   Ana

*This is an OneToOneField(Buy)

What I really need is to query a sum of the buy_price and sell_price that are NOT listed in Sell model (my_buy_object.sell.buyer == '')

I tryied the code below but it's not working properly:

buys = Buy.objects.order_by('-id')
sum_price = [buy.buy_price for buy in buys if buy.sell.buyer == '']
sum_price = sum(sum_price)

The correct answer for this code should be sum_buy_price: 110 and sum_sell_price: 360

Thank you!

EDIT: added the models:

class Buy(models.Model):
   category = models.ForeignKey(Category, on_delete=models.CASCADE)
   image = models.FileField()
   date = models.DateField(default=timezone.now)
   buy_price = models.DecimalField(max_digits=6, decimal_places=2)
   sell_price = models.DecimalField(max_digits=6, decimal_places=2)

   def __str__(self):
       return f'{self.id}'

class Sell(models.Model):
   item = models.OneToOneField(Buy, related_name='sell', 
   on_delete=models.CASCADE)
   date = models.DateField(default=timezone.now)
   discount = models.DecimalField(max_digits=6, decimal_places=2)
   total_paid = models.DecimalField(max_digits=6, decimal_places=2)
   buyer = models.ForeignKey(Buyer, related_name='sell', 
   on_delete=models.CASCADE)

   def __str__(self):
       return f'{self.buyer}'

can you add your models here ?

Astik Anand
– Astik Anand

2020-05-14 01:43:15 +00:00
Commented May 14, 2020 at 1:43 — Astik Anand
– Astik Anand, Commented May 14, 2020 at 1:43
Done, I've just added the models!

Felipe Dourado
– Felipe Dourado

2020-05-14 02:06:29 +00:00
Commented May 14, 2020 at 2:06 — Felipe Dourado
– Felipe Dourado, Commented May 14, 2020 at 2:06

JPG · Accepted Answer · 2020-05-14 02:10:39Z

1

To filter empty related fields, use isnull filter

from django.db.models import Sum

Buy.objects.filter(sell__buyer__isnull=True
                   ).aggregate(sum_buy_price=Sum('buy_price'),
                               sum_sell_price=Sum('sell_price'))

answered May 14, 2020 at 2:10

JPG

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1 Comment

Felipe Dourado Over a year ago

Thank you so much! It was exactly what I was expecting!

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