paperwork: Fix CertificationVersion is_latest/is_current when filtered

Unsurprisingly, filtering breaks window functions. Subqueries are less
elegant, but actually work :)

paperwork/models.py

@@ -5,8 +5,7 @@ from typing import TYPE_CHECKING, TypedDict
 from django.conf import settings
 from django.core.validators import RegexValidator
 from django.db import models
-from django.db.models import Q, Window
-from django.db.models.functions import FirstValue
+from django.db.models import OuterRef, Q, Subquery
 
 from django_stubs_ext import WithAnnotations
 from semver import VersionInfo
@@ -142,23 +141,18 @@ class CertificationVersionAnnotations(TypedDict):
 
 class CertificationVersionManager(models.Manager["CertificationVersion"]):
     def get_queryset(self) -> models.QuerySet["CertificationVersion"]:
-        window = {
-            "partition_by": "definition",
-            "order_by": [
-                "-major",
-                "-minor",
-                "-patch",
-                "-prerelease",
-            ],
-        }
-        return (
-            super()
-            .get_queryset()
-            .annotate(
-                is_latest=Q(pk=Window(FirstValue("pk"), **window)),
-                # TODO: should do a more correct comparison than just major version
-                is_current=Q(major=Window(FirstValue("major"), **window)),
-            )
+        qs = super().get_queryset()
+        department_versions = qs.filter(definition=OuterRef("definition")).order_by(
+            "-major",
+            "-minor",
+            "-patch",
+            "-prerelease",
+        )
+
+        return qs.annotate(
+            is_latest=Q(pk=Subquery(department_versions.values("pk")[:1])),
+            # TODO: should do a more correct comparison than just major version
+            is_current=Q(major=Subquery(department_versions.values("major")[:1])),
         )