[5.1.x] Fixed #35665 -- Fixed a crash when passing an empty order_by to Window.

This also caused un-ordered sliced prefetches to crash as they rely on Window.

Regression in e16d0c176e9b89628cdec5e58c418378c4a2436a that made OrderByList
piggy-back ExpressionList without porting the empty handling that the latter
provided.

Supporting explicit empty ordering on Window functions and slicing is arguably
a foot-gun design due to how backends will return undeterministic results but
this is a problem that requires a larger discussion.

Refs #35064.

Thanks Andrew Backer for the report and Mariusz for the review.

Backport of 602fe961e6834d665f2359087a1272e9f9806b71 from main.

django/db/backends/oracle/features.py

@@ -116,6 +116,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
             "Oracle requires ORDER BY in row_number, ANSI:SQL doesn't.": {
                 "expressions_window.tests.WindowFunctionTests."
                 "test_row_number_no_ordering",
+                "prefetch_related.tests.PrefetchLimitTests.test_empty_order",
             },
             "Oracle doesn't support changing collations on indexed columns (#33671).": {
                 "migrations.test_operations.OperationTests."

django/db/models/expressions.py

@@ -1377,16 +1377,14 @@ class ExpressionList(Func):
 
     template = "%(expressions)s"
 
-    def __init__(self, *expressions, **extra):
-        if not expressions:
-            raise ValueError(
-                "%s requires at least one expression." % self.__class__.__name__
-            )
-        super().__init__(*expressions, **extra)
-
     def __str__(self):
         return self.arg_joiner.join(str(arg) for arg in self.source_expressions)
 
+    def as_sql(self, *args, **kwargs):
+        if not self.source_expressions:
+            return "", ()
+        return super().as_sql(*args, **kwargs)
+
     def as_sqlite(self, compiler, connection, **extra_context):
         # Casting to numeric is unnecessary.
         return self.as_sql(compiler, connection, **extra_context)

docs/releases/5.1.1.txt

@@ -9,4 +9,6 @@ Django 5.1.1 fixes several bugs in 5.1.
 Bugfixes
 ========
 
-* ...
+* Fixed a regression in Django 5.1 that caused a crash of ``Window()`` when
+  passing an empty sequence to the ``order_by`` parameter, and a crash of
+  ``Prefetch()`` for a sliced queryset without ordering (:ticket:`35665`).

tests/expressions/tests.py

@@ -2311,11 +2311,6 @@ class ValueTests(TestCase):
         self.assertNotEqual(value, other_value)
         self.assertNotEqual(value, no_output_field)
 
-    def test_raise_empty_expressionlist(self):
-        msg = "ExpressionList requires at least one expression"
-        with self.assertRaisesMessage(ValueError, msg):
-            ExpressionList()
-
     def test_compile_unresolved(self):
         # This test might need to be revisited later on if #25425 is enforced.
         compiler = Time.objects.all().query.get_compiler(connection=connection)

tests/expressions_window/tests.py

@@ -928,6 +928,20 @@ class WindowFunctionTests(TestCase):
             ),
         )
 
+    def test_empty_ordering(self):
+        """
+        Explicit empty ordering makes little sense but it is something that
+        was historically allowed.
+        """
+        qs = Employee.objects.annotate(
+            sum=Window(
+                expression=Sum("salary"),
+                partition_by="department",
+                order_by=[],
+            )
+        ).order_by("department", "sum")
+        self.assertEqual(len(qs), 12)
+
     def test_related_ordering_with_count(self):
         qs = Employee.objects.annotate(
             department_sum=Window(

tests/prefetch_related/tests.py

@@ -1999,6 +1999,21 @@ class PrefetchLimitTests(TestDataMixin, TestCase):
         with self.assertRaisesMessage(NotSupportedError, msg):
             list(Book.objects.prefetch_related(Prefetch("authors", authors[1:])))
 
+    @skipUnlessDBFeature("supports_over_clause")
+    def test_empty_order(self):
+        authors = Author.objects.order_by()
+        with self.assertNumQueries(3):
+            books = list(
+                Book.objects.prefetch_related(
+                    Prefetch("authors", authors),
+                    Prefetch("authors", authors[:1], to_attr="authors_sliced"),
+                )
+            )
+        for book in books:
+            with self.subTest(book=book):
+                self.assertEqual(len(book.authors_sliced), 1)
+                self.assertIn(book.authors_sliced[0], list(book.authors.all()))
+
 
 class DeprecationTests(TestCase):
     def test_get_current_queryset_warning(self):