Ensure the app required for the database backend is installed. (#53)

Although the proper setup is documented in the README, it is easy to miss and (a little) hassle to debug without knowledge of the django_tasks code itself.

django_tasks/backends/database/backend.py

@@ -1,6 +1,8 @@
 from dataclasses import asdict, dataclass
-from typing import TYPE_CHECKING, TypeVar
+from typing import TYPE_CHECKING, Any, List, TypeVar
 
+from django.apps import apps
+from django.core.checks import ERROR, CheckMessage
 from django.core.exceptions import ValidationError
 from typing_extensions import ParamSpec
 
@@ -88,3 +90,17 @@ async def aget_result(self, result_id: str) -> TaskResult:
             return (await DBTaskResult.objects.aget(id=result_id)).task_result
         except (DBTaskResult.DoesNotExist, ValidationError) as e:
             raise ResultDoesNotExist(result_id) from e
+
+    def check(self, **kwargs: Any) -> List[CheckMessage]:
+        if not apps.is_installed("django_tasks.backends.database"):
+            backend_name = self.__class__.__name__
+
+            return [
+                CheckMessage(
+                    ERROR,
+                    f"{backend_name} configured as django_tasks backend, but database app not installed",
+                    "Insert 'django_tasks.backends.database' in INSTALLED_APPS",
+                )
+            ]
+
+        return []

tests/tests/test_database_backend.py

@@ -175,6 +175,18 @@ def test_missing_task_path(self) -> None:
         ):
             _ = db_task_result.task
 
+    def test_check(self) -> None:
+        errors = default_task_backend.check()
+
+        self.assertEqual(len(errors), 0)
+
+    @override_settings(INSTALLED_APPS=[])
+    def test_database_backend_app_missing(self) -> None:
+        errors = default_task_backend.check()
+
+        self.assertEqual(len(errors), 1)
+        self.assertIn("django_tasks.backends.database", errors[0].hint)
+
 
 @override_settings(
     TASKS={