Fixed #21181 -- Added Collate database function.

Thanks Simon Charette for reviews.

tests/db_functions/comparison/test_collate.py

@@ -0,0 +1,56 @@
+from django.db import connection
+from django.db.models import F, Value
+from django.db.models.functions import Collate
+from django.test import TestCase
+
+from ..models import Author
+
+
+class CollateTests(TestCase):
+    @classmethod
+    def setUpTestData(cls):
+        cls.author1 = Author.objects.create(alias='a', name='Jones 1')
+        cls.author2 = Author.objects.create(alias='A', name='Jones 2')
+
+    def test_collate_filter_ci(self):
+        collation = connection.features.test_collations.get('ci')
+        if not collation:
+            self.skipTest(
+                'This backend does not support case-insensitive collations.'
+            )
+        qs = Author.objects.filter(alias=Collate(Value('a'), collation))
+        self.assertEqual(qs.count(), 2)
+
+    def test_collate_order_by_cs(self):
+        collation = connection.features.test_collations.get('cs')
+        if not collation:
+            self.skipTest(
+                'This backend does not support case-sensitive collations.'
+            )
+        qs = Author.objects.order_by(Collate('alias', collation))
+        self.assertSequenceEqual(qs, [self.author2, self.author1])
+
+    def test_language_collation_order_by(self):
+        collation = connection.features.test_collations.get('swedish-ci')
+        if not collation:
+            self.skipTest('This backend does not support language collations.')
+        author3 = Author.objects.create(alias='O', name='Jones')
+        author4 = Author.objects.create(alias='Ö', name='Jones')
+        author5 = Author.objects.create(alias='P', name='Jones')
+        qs = Author.objects.order_by(Collate(F('alias'), collation), 'name')
+        self.assertSequenceEqual(
+            qs,
+            [self.author1, self.author2, author3, author5, author4],
+        )
+
+    def test_invalid_collation(self):
+        tests = [
+            None,
+            '',
+            'et-x-icu" OR ',
+            '"schema"."collation"',
+        ]
+        msg = "Invalid collation name: %r."
+        for value in tests:
+            with self.subTest(value), self.assertRaisesMessage(ValueError, msg % value):
+                Collate(F('alias'), value)