How to make sum query with type casting and calculation in django views?

问题

I'm calculating the sold items cost in django views and in django signals, and I want to calculate sold items cost on the fly. Price and quantity fields are integers. How can I convert one of them to the float and make sum query with some calculations like these sql queries below?

SELECT sum((t.price::FLOAT * t.quantity) / 1000) as cost FROM public."sold" t;

SELECT
t.id, t.price, t.quantity, sum((price::FLOAT * quantity) / 1000) as cost
FROM public."sold" t
GROUP BY t.id;

EDIT: Of course expected results are querysets of django

I expected the output of first query

             cost
-----------------
5732594.000000002

and I expected the output of second query

 id    price  quantity    cost
------------------------------
846     1100      5000    5500
790     1500      1000    1500
828     2600      1000    2600
938     1000      5000    5000
753     1500      2000    3000
652     5000      1520    7600

EDIT 2: I solved this issue via raw() method like

MyModel.objects.raw(
    'SELECT sum((t.price::FLOAT * t.quantity) / 1000) as cost '
    'FROM public."sold" t'
)

instead of pythonic way

回答1:

You'll need to have a look into a couple of things to do that. The first one will be aggregation and annotation. Also, you'll need to look into Cast and F functions. Please have a look at the links below:

https://docs.djangoproject.com/en/2.2/topics/db/aggregation/

https://docs.djangoproject.com/en/2.2/ref/models/database-functions/#cast

https://docs.djangoproject.com/en/2.2/ref/models/expressions/

DISCLAIMER: This is an example and might not work

Your queryset will look something like this:

from django.db.models import FloatField, Sum
from django.db.models.functions import Cast

qs = MyModel.objects.annotate(cost=Sum(F(Cast('price', FloatField())) * F('quantity') / 1000))

来源：https://stackoverflow.com/questions/56196815/how-to-make-sum-query-with-type-casting-and-calculation-in-django-views

标签

django

django-views

django-signals