Refs #28643 -- Added Reverse database function.

Thanks Mariusz Felisiak for Oracle advice and review.

tests/db_functions/text/test_reverse.py

@@ -0,0 +1,46 @@
+from django.db import connection
+from django.db.models import CharField
+from django.db.models.functions import Length, Reverse, Trim
+from django.test import TestCase
+from django.test.utils import register_lookup
+
+from ..models import Author
+
+
+class ReverseTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.john = Author.objects.create(name='John Smith', alias='smithj')
+        cls.elena = Author.objects.create(name='Élena Jordan', alias='elena')
+        cls.python = Author.objects.create(name='パイソン')
+
+    def test_null(self):
+        author = Author.objects.annotate(backward=Reverse('alias')).get(pk=self.python.pk)
+        self.assertEqual(author.backward, '' if connection.features.interprets_empty_strings_as_nulls else None)
+
+    def test_basic(self):
+        authors = Author.objects.annotate(backward=Reverse('name'))
+        self.assertQuerysetEqual(
+            authors,
+            [
+                ('John Smith', 'htimS nhoJ'),
+                ('Élena Jordan', 'nadroJ anelÉ'),
+                ('パイソン', 'ンソイパ'),
+            ],
+            lambda a: (a.name, a.backward),
+            ordered=False,
+        )
+
+    def test_transform(self):
+        with register_lookup(CharField, Reverse):
+            authors = Author.objects.all()
+            self.assertCountEqual(authors.filter(name__reverse=self.john.name[::-1]), [self.john])
+            self.assertCountEqual(authors.exclude(name__reverse=self.john.name[::-1]), [self.elena, self.python])
+
+    def test_expressions(self):
+        author = Author.objects.annotate(backward=Reverse(Trim('name'))).get(pk=self.john.pk)
+        self.assertEqual(author.backward, self.john.name[::-1])
+        with register_lookup(CharField, Reverse), register_lookup(CharField, Length):
+            authors = Author.objects.all()
+            self.assertCountEqual(authors.filter(name__reverse__length__gt=7), [self.john, self.elena])
+            self.assertCountEqual(authors.exclude(name__reverse__length__gt=7), [self.python])