Fixed #20429 -- Added QuerySet.update_or_create

Thanks tunixman for the suggestion and Loic Bistuer for the review.

tests/get_or_create/tests.py

@@ -131,3 +131,68 @@ class GetOrCreateThroughManyToMany(TestCase):
         Tag.objects.create(text='foo')
         a_thing = Thing.objects.create(name='a')
         self.assertRaises(IntegrityError, a_thing.tags.get_or_create, text='foo')
+
+
+class UpdateOrCreateTests(TestCase):
+
+    def test_update(self):
+        Person.objects.create(
+            first_name='John', last_name='Lennon', birthday=date(1940, 10, 9)
+        )
+        p, created = Person.objects.update_or_create(
+            first_name='John', last_name='Lennon', defaults={
+                'birthday': date(1940, 10, 10)
+            }
+        )
+        self.assertFalse(created)
+        self.assertEqual(p.first_name, 'John')
+        self.assertEqual(p.last_name, 'Lennon')
+        self.assertEqual(p.birthday, date(1940, 10, 10))
+
+    def test_create(self):
+        p, created = Person.objects.update_or_create(
+            first_name='John', last_name='Lennon', defaults={
+                'birthday': date(1940, 10, 10)
+            }
+        )
+        self.assertTrue(created)
+        self.assertEqual(p.first_name, 'John')
+        self.assertEqual(p.last_name, 'Lennon')
+        self.assertEqual(p.birthday, date(1940, 10, 10))
+
+    def test_create_twice(self):
+        params = {
+            'first_name': 'John',
+            'last_name': 'Lennon',
+            'birthday': date(1940, 10, 10),
+        }
+        Person.objects.update_or_create(**params)
+        # If we execute the exact same statement, it won't create a Person.
+        p, created = Person.objects.update_or_create(**params)
+        self.assertFalse(created)
+
+    def test_integrity(self):
+        # If you don't specify a value or default value for all required
+        # fields, you will get an error.
+        self.assertRaises(IntegrityError,
+            Person.objects.update_or_create, first_name="Tom", last_name="Smith")
+
+    def test_mananual_primary_key_test(self):
+        # If you specify an existing primary key, but different other fields,
+        # then you will get an error and data will not be updated.
+        ManualPrimaryKeyTest.objects.create(id=1, data="Original")
+        self.assertRaises(IntegrityError,
+            ManualPrimaryKeyTest.objects.update_or_create, id=1, data="Different"
+        )
+        self.assertEqual(ManualPrimaryKeyTest.objects.get(id=1).data, "Original")
+
+    def test_error_contains_full_traceback(self):
+        # update_or_create should raise IntegrityErrors with the full traceback.
+        # This is tested by checking that a known method call is in the traceback.
+        # We cannot use assertRaises/assertRaises here because we need to inspect
+        # the actual traceback. Refs #16340.
+        try:
+            ManualPrimaryKeyTest.objects.update_or_create(id=1, data="Different")
+        except IntegrityError as e:
+            formatted_traceback = traceback.format_exc()
+            self.assertIn('obj.save', formatted_traceback)