[1.8.x] Refs #24282 -- Added failing test case for assigning models of wrong type to FK

Thanks Jeff Singer for the test case.

Backport of 273bc4b667 from master

tests/migrations/test_operations.py

@@ -1659,6 +1659,60 @@ class OperationTests(OperationTestBase):
         self.assertEqual(definition[1], [])
         self.assertEqual(sorted(definition[2]), ["atomic", "code"])
 
+    def test_run_python_related_assignment(self):
+        """
+        #24282 - Tests that model changes to a FK reverse side update the model
+        on the FK side as well.
+        """
+
+        def inner_method(models, schema_editor):
+            Author = models.get_model("test_authors", "Author")
+            Book = models.get_model("test_books", "Book")
+            author = Author.objects.create(name="Hemingway")
+            Book.objects.create(title="Old Man and The Sea", author=author)
+
+        create_author = migrations.CreateModel(
+            "Author",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("name", models.CharField(max_length=100)),
+            ],
+            options={},
+        )
+        create_book = migrations.CreateModel(
+            "Book",
+            [
+                ("id", models.AutoField(primary_key=True)),
+                ("title", models.CharField(max_length=100)),
+                ("author", models.ForeignKey("test_authors.Author"))
+            ],
+            options={},
+        )
+        add_hometown = migrations.AddField(
+            "Author",
+            "hometown",
+            models.CharField(max_length=100),
+        )
+        create_old_man = migrations.RunPython(inner_method, inner_method)
+
+        project_state = ProjectState()
+        new_state = project_state.clone()
+        with connection.schema_editor() as editor:
+            create_author.state_forwards("test_authors", new_state)
+            create_author.database_forwards("test_authors", editor, project_state, new_state)
+            project_state = new_state
+            new_state = new_state.clone()
+            create_book.state_forwards("test_books", new_state)
+            create_book.database_forwards("test_books", editor, project_state, new_state)
+            project_state = new_state
+            new_state = new_state.clone()
+            add_hometown.state_forwards("test_authors", new_state)
+            add_hometown.database_forwards("test_authors", editor, project_state, new_state)
+            project_state = new_state
+            new_state = new_state.clone()
+            create_old_man.state_forwards("test_books", new_state)
+            create_old_man.database_forwards("test_books", editor, project_state, new_state)
+
     def test_run_python_noop(self):
         """
         #24098 - Tests no-op RunPython operations.