Django Conditional update based on Foreign key values / joined fields - django

Question

I'm trying to do a conditional update based on the value of a field on a foreign key. Example:
Model Kid: id, parent (a foreign key to Parent), has_rich_parent
Model Parent: id, income
So say I have a query set of A. I wanna update each item's has_guardian in A based on the value of age on the Kid's parent in one update. What I was trying to do is
queryset_of_kids.update(
has_rich_parent=Case(
When(parent__income__gte=10, then=True)
default=False
)
)
But this is giving me an error Joined field references are not permitted in this query. Which I am understanding it as joined fields / pursuing the foreignkey relationships aren't allowed in updates.
I'm wondering if there's any other way to accomplish the same thing, as in updating this queryset within one update call? My situation has a couple more fields that I'd like to verify instead of just income here so if I try to do filter then update, the number of calls will be linear to the number of arguments I'd like to filter/update.
Thanks in advance!

Answer 1

Here are the models that I assume you're using:
from django.db import models
class Kid(models.Model):
parent = models.ForeignKey('Parent', on_delete=models.CASCADE)
has_rich_parent = models.BooleanField(default=False)
class Parent(models.Model):
income = models.IntegerField()
You can use a Subquery to update the has_rich_parent field.
The subquery filters on the primary key pk of the surrounding query using .filter(pk=OuterRef('pk')).
It uses a Q query object to obtain whether the parent income is >= 10.
from .models import Kid, Parent
from django.db.models import Q, Subquery, OuterRef
Kid.objects.update(has_rich_parent=Subquery(
Kid.objects.filter(pk=OuterRef('pk'))
.values_list(Q(parent__income__gte=10))))
That command produces the following SQL query:
UPDATE "more_kids_kid"
SET "has_rich_parent" = (
SELECT (U1."income" >= 10) AS "q1"
FROM "more_kids_kid" U0
INNER JOIN "more_kids_parent" U1 ON (U0."parent_id" = U1."id")
WHERE U0."id" = ("more_kids_kid"."id")
)
This query isn't as efficient as a SELECT-then-UPDATE query. However, your database may be able to optimize it.