Django queryset field replaced with related table field - sql

Question

I have 2 tables (Product and CustomerProduct)
CustomerProduct is the intermediate table between Customer and Product. It assigns customer specific pricing to certain products.
Product Model
class Product(models.Model):
name = models.CharField(max_length=200)
price = models.DecimalField(max_digits=12, decimal_places=2)
sample data
id
name
price
1
orange
1.5
2
apple
2.2
3
kiwi
3.5
CustomerProduct Model
class CustomerProduct(models.Model):
customer = models.ForeignKey(
"Customer",
on_delete=models.CASCADE,
related_name="customer_customerproducts",
)
product = models.ForeignKey(
"Product",
on_delete=models.CASCADE,
related_name="product_customerproducts",
)
price = models.DecimalField(
max_digits=12,
decimal_places=2,
)
sample data
id
customer_id
product_id
price
1
233
1
1.2
2
233
2
2.0
Expected Result
I want to query the all products but the Product.price adjusted according to CustomerProduct.price if related field exists. The expected data (example in json but want queryset):
[
{
id: 1
name: "orange"
price: 1.2 // The price adjusted according to customer price
}
{
id: 2
name: "apple"
price: 2.0 // The price adjusted according to customer price
}
{
id: 3
name: "kiwi"
price: 3.5 // Price remain original because CustomerProduct not exists.
}
]
The How
I totally have no idea how to accomplish this in Django. How to do that?

Answer 1

You can use Coalesce [Django docs] with Subquery expressions [Django docs] to annotate a value on your queryset which will be the price that you want. Coalesce gives the first non-null value of the expressions passed to it, so we will pass it a subquery that will get the related price from CustomerProduct or the field price itself in that order to achieve what you want:
from django.db.models import F, OuterRef, Subquery
from django.db.models.functions import Coalesce
customer = Customer.objects.get(pk=some_pk) # get the customer for which you want the results
# Last related price, will use it as a subquery
related_price = CustomerProduct.objects.filter(
product=OuterRef('pk'),
customer=customer,
).order_by('-pk').values('price')[:1]
products = Product.objects.annotate(
price_offered=Coalesce(
Subquery(related_price), # If there is a matching object in the subquery use it
F('price'), # Else use the price of the Product itself
)
)
for product in products:
print(product.pk, product.name, product.price_offered)