`update_change_reason`: fails to fetch latest history entry for nullable fields

[Senor-Ducky]

Describe the bug
The update_change_reason method in utils.py fails to update change reason in case any field is nullable in a model or is set to default to None. This could be due to the way django handles null values, as direct comparison to None is not allowed when filtering.

To Reproduce
Steps to reproduce the behavior:

1. Create a model with any nullable field or a field that defaults to None and enable historical records.
2. Try to use the update_change_reason on an instance of the model.

Expected behavior
The update_change_reason should handle models with nullable fields/fields with default set to None gracefully by applying the correct filter query for fetching historical records.

Screenshots

Environment (please complete the following information):

• OS: Win 11 24H2
• Django Simple History Version: 3.8.0
• Django Version: 5.0.11
• Database Version: PostgreSQL 16.8

Additional context
In the provided screenshot you can see, the History Attrs logged from within the update_change_reason method indicating that it is using the wrong filter query for filtering/matching null value, instead of field=None it should have been field__isnull=True, which prevented the historical record from showing up causing an exception.

[ddabble]

This could be due to the way django handles null values, as direct comparison to None is not allowed when filtering.
[...]
indicating that it is using the wrong filter query for filtering/matching null value, instead of field=None it should have been field__isnull=True

?? Django handles filter(field=None) in exactly the same way as filter(field__isnull=True) (see the queryset docs).

Also, the test you added in your PR passes without the other changes in the PR 😅

Now, looking at the screenshot you posted, it seems like the actual issue is that the instance you tried to update history_change_reason for didn't have any historical records. Could you try verifying whether that's the case? 🙂

[Senor-Ducky]

This could be due to the way django handles null values, as direct comparison to None is not allowed when filtering.
[...]
indicating that it is using the wrong filter query for filtering/matching null value, instead of field=None it should have been field__isnull=True

?? Django handles filter(field=None) in exactly the same way as filter(field__isnull=True) (see the queryset docs).

Also, the test you added in your PR passes without the other changes in the PR 😅

Now, looking at the screenshot you posted, it seems like the actual issue is that the instance you tried to update history_change_reason for didn't have any historical records. Could you try verifying whether that's the case? 🙂

Yes this is the case, it didn't have any historical records previously.

[ddabble]

Ah, I see. We should definitely make update_change_reason() let the caller know if instance has no historical records, to help with using the function 🙂

Closing the issue, now that we found the root cause.