Failure when using expressions.DateTime on NULL values and aggregating

[Thomas Recouvreux]

Running query like qs.annotate(time=DateTime('removed'), 'month', timezone.UTC()).values('time').annotate(Count('pk')) fails with <repr(<gill.contrib.sales.models.TransactionRowQuerySet at 0x10cc99320>) failed: ValueError: Database returned an invalid value in QuerySet.datetimes(). Are time zone definitions for your database and pytz installed?>. I noted the failure on MySQL and PostgreSQL, Django 1.8.X and 1.9.X. I did not try on other db backends or other Django versions.

To reproduce the behaviour:

1. create a model:

   class Potato(models.Model):
       ..
       removed = models.DateTimeField(null=True, blank=True)
   

2. Insert some of them

   Potato.objects.create(removed=timezone.now())
   Potato.objects.create(removed=None)
   ..
   

3. Fire the request

   # Failure
   Potato.annotate(time=DateTime('removed'), 'month', timezone.UTC()).values('time').annotate(c=Count('pk'))
   
   # Success
   Potato.filter(removed__isnull=False).annotate(time=DateTime('removed'), 'month', timezone.UTC()).values('time').annotate(c=Count('pk'))
   

Note: I printed the raw sql generated by Django and fired it directly in a db shell without any error, so the problem seems to come from the code Django uses to convert the result from the backend to python code, especially the line with NULL coming from the aggregation:

+---------------------+-----------+
| time                | pk__count |
+---------------------+-----------+
| NULL                |        17 |
| 2015-01-01 00:00:00 |         7 |
..