Add test for enqueuing async task on incompatible backend

Author: RealOrangeOne

tests/tests/test_custom_backend.py

@@ -0,0 +1,36 @@
+from typing import Any
+from unittest import mock
+
+from django.test import SimpleTestCase, override_settings
+
+from django_tasks import default_task_backend, tasks
+from django_tasks.backends.base import BaseTaskBackend
+from django_tasks.exceptions import InvalidTaskError
+from django_tasks.utils import get_module_path
+from tests import tasks as test_tasks
+
+
+class CustomBackend(BaseTaskBackend):
+    def enqueue(self, *args: Any, **kwargs: Any) -> Any:
+        pass
+
+
+@override_settings(
+    TASKS={
+        "default": {
+            "BACKEND": get_module_path(CustomBackend),
+            "ENQUEUE_ON_COMMIT": False,
+        }
+    }
+)
+class CustomBackendTestCase(SimpleTestCase):
+    def test_using_correct_backend(self) -> None:
+        self.assertEqual(default_task_backend, tasks["default"])
+        self.assertIsInstance(tasks["default"], CustomBackend)
+
+    @mock.patch.multiple(CustomBackend, supports_async_task=False)
+    def test_enqueue_async_task_on_non_async_backend(self) -> None:
+        with self.assertRaisesMessage(
+            InvalidTaskError, "Backend does not support async tasks"
+        ):
+            default_task_backend.validate_task(test_tasks.noop_task_async)