Exceptions using date-based generic views with MySQL, USE_TZ=True and bad data

[Chris Adams]

I ran into a non-obvious exception while using the date-based generic views. get_previous_day / get_next_day were raising an AttributeError ('NoneType' object has no attribute 'astimezone') in the following code when the view was called with the earliest item in the system:

        try:
            result = getattr(qs[0], date_field)
        except IndexError:
            return None

        # Convert datetimes to dates in the current time zone.
        if generic_view.uses_datetime_field:
            if settings.USE_TZ:
                result = timezone.localtime(result)
            result = result.date()

This error wasn't immediately obvious because the query shouldn't have been able to return a NULL. Looking at the data revealed the problem: a small number of records on my test instance had invalid dates in MySQL (0000-00-00) which were included by the lt query but subsequently converted into None by the ORM.

The nature of the bug suggests that the cleanest patch might simply be an immediate "if not result: return None", perhaps with a warning about invalid data.

[Aymeric Augustin]

We discussed this ticket on IRC. I believe it's an acceptable solution, provided there's an adequate comment and a MySQL-specific test. The code isn't complex, so this change is low risk.