How can i do this via aggregation in django-CodePudding

I have a method in the model that saves the final price in the cart, it looks like this:

class Cart(models.Model):
    """Cart"""

    owner = models.OneToOneField('Customer', on_delete=models.CASCADE)
    meals = models.ManyToManyField(CartMeal, related_name='related_cart', blank=True)
    total_products = models.PositiveIntegerField(default=0)
    final_price = models.DecimalField(max_digits=9, decimal_places=2, default=0)
    in_orders = models.BooleanField(default=False)
    for_anonymous_user = models.BooleanField(default=False)


    def save(self, *args, **kwargs):
        if self.id:
            self.total_products = self.meals.count()
            self.final_price = sum([cmeals.final_price for cmeals in self.meals.all()])
        super().save(*args, **kwargs)

I was told that I can make this line self.final_price = sum([cmeals.final_price for cmeals in self.meals.all()]) with a single query using aggregate.

How can I do this and where? In the model or do I need to do this in the view? Thanks.

CodePudding user response：

Cart.objects.aggregate(
    final_price=Sum(F('meals__final_price'), 
    output_field=IntegerField()
)
# returns ->
{'final_price': 10}

I would suggest not to store it in db, but create a property in model

@property
def final_price(self):
    return self.meals.aggregate(
        final_price=Sum(F('final_price'), 
        output_field=IntegerField()
    )["final_price"]

It can be accessed same as field:

my_cart.final_price