Annotate a sum of two fields multiplied

Question

I have three models, simplified for the example:

class Customer(models.Model):
    email = models.CharField(max_length=128)

class Order(models.Model):
    custo         


        

Answer 1

Have you looked at using the .extra() method?

See the Django QuerySet API.

Answer 2

You'll probably need to roll out your own custom aggregator. You can find a simple walk through of a GROUP_CONCAT example that should get you started here: http://harkablog.com/inside-the-django-orm-aggregates.html

Answer 3

Similar to: https://stackoverflow.com/a/19888120/1344647

from django.db.models import Sum

q = Task.objects.filter(your-filter-here).annotate(total=Sum('progress', field="progress*estimated_days"))

Edited: Thanks to @Max, using annotate instead aggregated.

Answer 4

Maybe you don't need this answer now, but if you read the documentation about Sum expression , you need to declare the output_field, like this:

Customer.objects.filter(something)
                .annotate(total_spent=Sum(
                    F('order__lineitem__quantity') * 
                    F('order__lineitem__price'),   
                    output_field=models.FloatField()
                ))

Answer 5

I just ran into this and I don't think that annotate and will work with a property, see Django - Can you use property as the field in an aggregation function?

Here is what I did.

class Customer(models.Model):
    email = models.CharField(max_length=128)

class Order(models.Model):
    customer = models.ForeignKey(Customer)
    order_status = models.CharField(blank=True, max_length=256)

class Lineitem(models.Model):
    order = models.ForeignKey(Order)
    quantity = models.IntegerField(blank=True)
    price = models.DecimalField(max_digits=6, decimal_places=2)
    @property
    def total(self):
        return self.quantity * self.price

Then use sum and a list comprehension:

sum([li.total for li in  LineItem.objects.filter(order__customer=some_customer).filter(somefilter)])

Answer 6

You could try using a property in the LineItem model:

class Lineitem(models.Model):
    order = models.ForeignKey(Order)
    quantity = models.IntegerField(blank=True)
    price = models.DecimalField(max_digits=6, decimal_places=2)
    def _get_total(self):
        return quantity * price
    total = property(_get_total)

Then I think you can annotate with the total amount spent using

Customer.objects.filter(something).annotate(total_spent=Sum('order__lineitem__total'))

I don't know how the efficiency of this method relates to others, but it is more Pythonic/Django-y than the alternative, which is to write the entire SQL query by hand as in

Customer.objects.raw("SELECT ... from <customer_table_name> where ...")