Ensure return value is correctly exposed using database backend

It seems the return value was always `None`, regardless of if the task actually returned a value.

Fixes #131

django_tasks/backends/database/models.py

@@ -167,6 +167,7 @@ def task_result(self) -> "TaskResult[T]":
 
         object.__setattr__(task_result, "_exception_class", exception_class)
         object.__setattr__(task_result, "_traceback", self.traceback or None)
+        object.__setattr__(task_result, "_return_value", self.return_value)
 
         return task_result
 

tests/tests/test_database_backend.py

@@ -127,6 +127,7 @@ def test_refresh_result(self) -> None:
             status=ResultStatus.SUCCEEDED,
             started_at=timezone.now(),
             finished_at=timezone.now(),
+            return_value=42,
         )
 
         self.assertEqual(result.status, ResultStatus.NEW)
@@ -139,6 +140,7 @@ def test_refresh_result(self) -> None:
         self.assertIsNotNone(result.finished_at)
         self.assertEqual(result.status, ResultStatus.SUCCEEDED)
         self.assertTrue(result.is_finished)
+        self.assertEqual(result.return_value, 42)
 
     async def test_refresh_result_async(self) -> None:
         result = await default_task_backend.aenqueue(
@@ -149,6 +151,7 @@ async def test_refresh_result_async(self) -> None:
             status=ResultStatus.SUCCEEDED,
             started_at=timezone.now(),
             finished_at=timezone.now(),
+            return_value=42,
         )
 
         self.assertEqual(result.status, ResultStatus.NEW)
@@ -160,6 +163,7 @@ async def test_refresh_result_async(self) -> None:
         self.assertIsNotNone(result.finished_at)
         self.assertEqual(result.status, ResultStatus.SUCCEEDED)
         self.assertTrue(result.is_finished)
+        self.assertEqual(result.return_value, 42)
 
     def test_get_missing_result(self) -> None:
         with self.assertRaises(ResultDoesNotExist):