Fixed #33772 -- Added QuerySet.first()/last() error message on unordered queryset with aggregation.

django/db/models/query.py

@@ -1032,7 +1032,12 @@ class QuerySet:
 
     def first(self):
         """Return the first object of a query or None if no match is found."""
-        for obj in (self if self.ordered else self.order_by("pk"))[:1]:
+        if self.ordered:
+            queryset = self
+        else:
+            self._check_ordering_first_last_queryset_aggregation(method="first")
+            queryset = self.order_by("pk")
+        for obj in queryset[:1]:
             return obj
 
     async def afirst(self):
@@ -1040,7 +1045,12 @@ class QuerySet:
 
     def last(self):
         """Return the last object of a query or None if no match is found."""
-        for obj in (self.reverse() if self.ordered else self.order_by("-pk"))[:1]:
+        if self.ordered:
+            queryset = self.reverse()
+        else:
+            self._check_ordering_first_last_queryset_aggregation(method="last")
+            queryset = self.order_by("-pk")
+        for obj in queryset[:1]:
             return obj
 
     async def alast(self):
@@ -1926,6 +1936,15 @@ class QuerySet:
         if self.query.combinator or other.query.combinator:
             raise TypeError(f"Cannot use {operator_} operator with combined queryset.")
 
+    def _check_ordering_first_last_queryset_aggregation(self, method):
+        if isinstance(self.query.group_by, tuple) and not any(
+            col.output_field is self.model._meta.pk for col in self.query.group_by
+        ):
+            raise TypeError(
+                f"Cannot use QuerySet.{method}() on an unordered queryset performing "
+                f"aggregation. Add an ordering with order_by()."
+            )
+
 
 class InstanceCheckMeta(type):
     def __instancecheck__(self, instance):

tests/get_earliest_or_latest/models.py

@@ -16,6 +16,11 @@ class Person(models.Model):
     # Note that this model doesn't have "get_latest_by" set.
 
 
+class Comment(models.Model):
+    article = models.ForeignKey(Article, on_delete=models.CASCADE)
+    likes_count = models.PositiveIntegerField()
+
+
 # Ticket #23555 - model with an intentionally broken QuerySet.__iter__ method.
 
 

tests/get_earliest_or_latest/tests.py

@@ -1,8 +1,9 @@
 from datetime import datetime
 
+from django.db.models import Avg
 from django.test import TestCase
 
-from .models import Article, IndexErrorArticle, Person
+from .models import Article, Comment, IndexErrorArticle, Person
 
 
 class EarliestOrLatestTests(TestCase):
@@ -245,3 +246,21 @@ class TestFirstLast(TestCase):
             expire_date=datetime(2005, 9, 1),
         )
         check()
+
+    def test_first_last_unordered_qs_aggregation_error(self):
+        a1 = Article.objects.create(
+            headline="Article 1",
+            pub_date=datetime(2005, 7, 26),
+            expire_date=datetime(2005, 9, 1),
+        )
+        Comment.objects.create(article=a1, likes_count=5)
+
+        qs = Comment.objects.values("article").annotate(avg_likes=Avg("likes_count"))
+        msg = (
+            "Cannot use QuerySet.%s() on an unordered queryset performing aggregation. "
+            "Add an ordering with order_by()."
+        )
+        with self.assertRaisesMessage(TypeError, msg % "first"):
+            qs.first()
+        with self.assertRaisesMessage(TypeError, msg % "last"):
+            qs.last()

tests/gis_tests/geoapp/test_expressions.py

@@ -62,6 +62,7 @@ class GeoExpressionsTests(TestCase):
         )
         qs = (
             City.objects.values("name")
+            .order_by("name")
             .annotate(
                 distance=Min(
                     functions.Distance("multifields__point", multi_field.city.point)