Fixed #21961 -- Added support for database-level delete options for ForeignKey.

Thanks Simon Charette for pair programming.

Co-authored-by: Nick Stefan <NickStefan12@gmail.com>
Co-authored-by: Akash Kumar Sen <71623442+Akash-Kumar-Sen@users.noreply.github.com>
Co-authored-by: Simon Charette <charette.s@gmail.com>

tests/invalid_models_tests/test_models.py

@@ -3062,3 +3062,64 @@ class ConstraintsTests(TestCase):
                 ),
             ],
         )
+
+
+@isolate_apps("invalid_models_tests")
+class RelatedFieldTests(SimpleTestCase):
+    def test_on_delete_python_db_variants(self):
+        class Artist(models.Model):
+            pass
+
+        class Album(models.Model):
+            artist = models.ForeignKey(Artist, models.CASCADE)
+
+        class Song(models.Model):
+            album = models.ForeignKey(Album, models.RESTRICT)
+            artist = models.ForeignKey(Artist, models.DB_CASCADE)
+
+        self.assertEqual(
+            Song.check(databases=self.databases),
+            [
+                Error(
+                    "The model cannot have related fields with both database-level and "
+                    "Python-level on_delete variants.",
+                    obj=Song,
+                    id="models.E050",
+                ),
+            ],
+        )
+
+    def test_on_delete_python_db_variants_auto_created(self):
+        class SharedModel(models.Model):
+            pass
+
+        class Parent(models.Model):
+            pass
+
+        class Child(SharedModel):
+            parent = models.ForeignKey(Parent, on_delete=models.DB_CASCADE)
+
+        self.assertEqual(
+            Child.check(databases=self.databases),
+            [
+                Error(
+                    "The model cannot have related fields with both database-level and "
+                    "Python-level on_delete variants.",
+                    obj=Child,
+                    id="models.E050",
+                ),
+            ],
+        )
+
+    def test_on_delete_db_do_nothing(self):
+        class Artist(models.Model):
+            pass
+
+        class Album(models.Model):
+            artist = models.ForeignKey(Artist, models.CASCADE)
+
+        class Song(models.Model):
+            album = models.ForeignKey(Album, models.DO_NOTHING)
+            artist = models.ForeignKey(Artist, models.DB_CASCADE)
+
+        self.assertEqual(Song.check(databases=self.databases), [])