Fix atomic transaction not routing to the the correct DB

[nofalx]

This to fix the issue explained at: #323

[auvipy]

can you also add some tests for the change?

[nofalx]

I have added a new test class and a db router as these are required to test this specific scenario

using make test

The test passes after the edits to the manager.py file

before the changes I added to manager.py the test fails with the below error:

FAILED t/unit/test_models.py::test_ModelsOnSecondaryDbOnly::test_operations_with_atomic_transactions - AssertionError: Database connections to 'default' are not allowed in this test. Add 'default' to t.unit.test_models.test_ModelsOnSecondaryDbOnly.databases to ensure proper test isolation and silence this failure.

[auvipy]

can you explain this part a bit more please?

[nofalx]

We will try to run the method delete_expired from the Custom Manager ResultManager where the bug occurs.

The issue is that if the manager was directed to use a db other than the "default" db, the method delete_expired would run the operation on the "default" anyways. Since the function does not return anything and is a delete operation I can not use assert

My method of testing is the below: (let me know if there is a better way)

I specify the allowed db to test on as "secondary" only. If the test tries to use the "default" db it will throw error:
https://github.com/nofalx/django-celery-results/blob/fix-323/t/unit/test_models.py#L231:L232

I specify the TaskResult and the GroupResult manager which extend ResultManager to use "secondary" db using .db_manager("secondary").

https://github.com/nofalx/django-celery-results/blob/fix-323/t/unit/test_models.py#L235:L240

Before the fixes this test would fail as transaction.atomic() would try to use the "default" db. After the fix the method doesnt use "default" db anymore and the test pass.