Django attribute of most recent reverse relation - django

Question

I have two models:
class Test(models.Model):
test_id = models.CharField(max_length=20, unique=True, db_index=True)
class TestResult(models.Model):
test = models.ForeignKey("Test", to_field="test_id", on_delete=models.CASCADE)
status = models.CharField(max_length=30, choices=status_choices)
with status_choices as an enumeration of tuples of strings.
Some Test objects may have zero related TestResult objects, but most have at least one.
I want to filter Test objects based on their most recent TestResult status.
I have tried this:
queryset = Test.objects.all()
queryset = queryset.annotate(most_recent_result_pk=Max("testresult__pk"))
queryset = queryset.annotate(current_status=Subquery(TestResult.objects.filter(pk=OuterRef("most_recent_result")).values("status")[:1]))
But I get the error:
column "u0.status" must appear in the GROUP BY clause or be used in an
aggregate function LINE 1: ...lts_testresult"."id") AS
"most_recent_result_pk", (SELECT U0."status...
I can find the most recent TestResult object fine with the first annotation of the pk, but the second annotation breaks everything. It seems like it ought to be easy to find an attribute of the TestResult object, once its pk is known. How can I do this?

Answer 1

You can do this with one subquery, without annotating this first:
from django.db.models import OuterRef, Subquery
queryset = Test.objects.annotate(
current_status=Subquery(
TestResult.objects.filter(
test=OuterRef('pk')
).order_by('-pk').values('status')[:1])
)
This will generate a query that looks like:
SELECT test.*,
(SELECT U0.status
FROM testresult U0
WHERE U0.test_id = test.id
ORDER BY U0.id DESC
LIMIT 1
) AS current_status
FROM test
or without subquery:
from django.db.models import F, Max
queryset = Test.objects.annotate(
max_testresult=Max('testresult__test__testresult__pk')
).filter(
testresult__pk=F('max_testresult')
).annotate(
current_status=F('testresult__status')
)
That being said, ordering by primary key is not a good idea to retrieve the latest object. You can see primary keys as "blackboxes" that simply hold a value to refer to it.
It is often better to use a column that stores the timestamp:
class TestResult(models.Model):
test = models.ForeignKey("Test", to_field="test_id", on_delete=models.CASCADE)
status = models.CharField(max_length=30, choices=status_choices)
created = models.DateTimeField(auto_now_add=True)
and then query with:
from django.db.models import OuterRef, Subquery
queryset = Test.objects.annotate(
current_status=Subquery(
TestResult.objects.filter(
test=OuterRef('pk')
).order_by('-created').values('status')[:1])
)