How to annotate an object with the sum of multilplication of two fields from a related object to a realated object in Django

Question

Given these three classes

    class User(BaseModel):
          name = models.CharField(..)

    class Order(BaseModel):
          user = models.ForeignKey(User,...,related_name='orders')

  class OrderItem(BaseModel):
        order = models.ForeignKey(Order,...,related_name='items'
        quatity = models.IntegerField(default=1)
        price = models.FloatField()

and this is the base class (it is enough to note that it has the created_at field)

    class BaseModel(models.Model):
          createt_at = models.DateTimeField(auto_now_add=True)

Now each User will have multiple Orders and each Order has multiple OrdeItems

I want to annotate the User objects with the total price of the last order.

Take this data for example:

The User objects should be annotated with the sum of the last order that is for user john with id=1 we should return the sum of order_items (with ids= 3 & 4) since they are related to the order id=2 since it is the latest order.

I hope I have made my self clear. I am new to Django and tried to go over the docs and tried many different things but I keep getting stuck at getting the last order items

Answer 1

Sometimes it's unclear how to make such query in Django ORM. In your case I'd write the query in raw SQL something like:

WITH last_order_for_user AS (
    SELECT id, user_id, MAX(created_at)
    FROM orders
    GROUP BY user_id
) -- last order_id for each user_id
SELECT
    order.user_id, order.id, SUM(item.price)
FROM 
    last_order_for_user order 
LEFT JOIN 
    orderitems item ON order.id=item.order_id
GROUP BY 1,2 -- sum of items for last order, user

And then perform raw SQL django-docs