Fixed #36644 -- Enabled empty order_by() to avoid pk ordering by first()/last().

3 files changed, 39 insertions, 3 deletions

diff --git a/django/db/models/query.py b/django/db/models/query.py
index afb3aac5cb..76d0f449a6 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -1158,7 +1158,7 @@ class QuerySet(AltersData):
 
     def first(self):
         """Return the first object of a query or None if no match is found."""
-        if self.ordered:
+        if self.ordered or not self.query.default_ordering:
             queryset = self
         else:
             self._check_ordering_first_last_queryset_aggregation(method="first")
@@ -1171,7 +1171,7 @@ class QuerySet(AltersData):
 
     def last(self):
         """Return the last object of a query or None if no match is found."""
-        if self.ordered:
+        if self.ordered or not self.query.default_ordering:
             queryset = self.reverse()
         else:
             self._check_ordering_first_last_queryset_aggregation(method="last")
diff --git a/tests/get_earliest_or_latest/models.py b/tests/get_earliest_or_latest/models.py
index bbf2075d36..91725865b1 100644
--- a/tests/get_earliest_or_latest/models.py
+++ b/tests/get_earliest_or_latest/models.py
@@ -21,6 +21,14 @@ class Comment(models.Model):
     likes_count = models.PositiveIntegerField()
 
 
+class OrderedArticle(models.Model):
+    headline = models.CharField(max_length=100)
+    pub_date = models.DateField()
+
+    class Meta:
+        ordering = ["headline"]
+
+
 # Ticket #23555 - model with an intentionally broken QuerySet.__iter__ method.
 
 
diff --git a/tests/get_earliest_or_latest/tests.py b/tests/get_earliest_or_latest/tests.py
index 49c803b73a..793fb12bb9 100644
--- a/tests/get_earliest_or_latest/tests.py
+++ b/tests/get_earliest_or_latest/tests.py
@@ -1,9 +1,10 @@
 from datetime import datetime
+from unittest.mock import patch
 
 from django.db.models import Avg
 from django.test import TestCase
 
-from .models import Article, Comment, IndexErrorArticle, Person
+from .models import Article, Comment, IndexErrorArticle, OrderedArticle, Person
 
 
 class EarliestOrLatestTests(TestCase):
@@ -265,3 +266,30 @@ class TestFirstLast(TestCase):
             qs.first()
         with self.assertRaisesMessage(TypeError, msg % "last"):
             qs.last()
+
+    def test_first_last_empty_order_by_has_no_pk_ordering(self):
+        Article.objects.create(
+            headline="Article 1",
+            pub_date=datetime(2006, 9, 10),
+            expire_date=datetime(2056, 9, 11),
+        )
+
+        qs = Article.objects.order_by()
+        with patch.object(type(qs), "order_by") as mock_order_by:
+            qs.first()
+            mock_order_by.assert_not_called()
+            qs.last()
+            mock_order_by.assert_not_called()
+
+    def test_first_last_empty_order_by_clears_default_ordering(self):
+        OrderedArticle.objects.create(
+            headline="Article 1",
+            pub_date=datetime(2006, 9, 10),
+        )
+
+        qs = OrderedArticle.objects.order_by()
+        with patch.object(type(qs), "order_by") as mock_order_by:
+            qs.first()
+            mock_order_by.assert_not_called()
+            qs.last()
+            mock_order_by.assert_not_called()