Union of TruncBase annotations with different tzinfo apply `convert_value` of last tzinfo.

[Jurgis Pralgauskis]

class Message(models.Model):
    timestamp = models.DateTimeField(auto_now=True) # PostgreSQL, USE_TZ=True
    msg = models.CharField(max_length=254)
    timezone = models.CharField(max_length=64, default="UTC", help_text="pytz name")

    def test_demo_bug(self):
        from ..models import Message
        import pytz
        from django.db.models.functions import Trunc, TruncSecond
        import mock
        from django.utils import timezone

        with patch.object(timezone, 'now', return_value=pytz.utc.localize(datetime(2017, 1, 1, 0, 0))):

            # we have info
            Message.objects.create( msg = "bar", timezone = 'UTC')
            Message.objects.create( msg = "foo", timezone = 'America/Los_Angeles')


            # we want to get it with "localized" timestamps:
            qs = Message.objects.all()

            partitions = []  # partition by timezones
            for tzname in ['UTC', 'America/Los_Angeles']:
                tz = pytz.timezone(tzname)
                _qs = qs.filter(timezone=tzname)
                _qs = _qs.annotate(trunc_local_time=TruncSecond('timestamp', tzinfo=tz)) # could be Trunc_anything_
                partitions.append(_qs)

            qs1, qs2 = partitions

            result = list(qs1.union(qs2))

            for x in result:
                tz = pytz.timezone(x.timezone)
                print(x.msg)
                x.timestamp_trunc = x.timestamp.replace(microsecond=0)
                x.expected = x.timestamp_trunc.astimezone(tz)  # this is the working way  (but I wanted to get localization in DB layer)
                assert x.trunc_local_time == x.expected,  "Error (msg: '{x.msg}'): {x.trunc_local_time} != {x.expected}".format(x=x)

Failure
Traceback (most recent call last):
  File "/home/jurgis/dev/new/tableair/sync_tableair-cloud/ta/api/booking/tests/test_endpoints.py", line 1107, in test_demo_bug
    assert x.trunc_local_time == x.expected,  "Error (msg: '{x.msg}'): {x.trunc_local_time} != {x.expected}".format(x=x)
AssertionError: Error (msg: 'bla'): 2016-12-31 16:00:00+00:00 != 2017-01-01 00:00:00+00:00}}}

[Jurgis Pralgauskis]

related idea, using CASE/WHEN, gives localized (but naive) datetime (because it just don't apply BaseTrunc.convert_value
​https://mjec.blog/2016/06/27/djangos-queryset-union-isnt-quite-what-it-seems/

[Jurgis Pralgauskis]

workarounds are:

• to take initial query and later adapt

  tz = pytz.timezone(x.timezone)
  x.timestamp.astimezone(tz)
  

• or just make separate queries per timezone and chain (kind of union) them in python

[Mariusz Felisiak]

Thanks for the report, however ticket description is not correct. Django uses tzinfo properly in your use case:

SELECT
    ...
    DATE_TRUNC('second', "ticket_30575_message"."timestamp" AT TIME ZONE 'UTC') AS "trunc_local_time",
    UTC AS "tzname"
FROM "ticket_30575_message" WHERE "ticket_30575_message"."timezone" = UTC
UNION
SELECT
    ...
    DATE_TRUNC('second', "ticket_30575_message"."timestamp" AT TIME ZONE 'America/Los_Angeles') AS "trunc_local_time",
    America/Los_Angeles AS "tzname"
FROM "ticket_30575_message" WHERE "ticket_30575_message"."timezone" = UTC)

i.e. UTC in a qs1 and America/Los_Angeles in qs2. You can add tzname to you queryset (e.g. tzname=Value(tzname, output_field=CharField())) to check that everything works properly:

>>> for x in result:
>>>     print(x.timestamp, x.trunc_local_time, x.tzname)
2019-06-18 10:07:04.111245+00:00 2019-06-18 03:07:04+00:00 America/Los_Angeles 
2019-06-18 10:07:04.111245+00:00 2019-06-18 10:07:04+00:00 UTC 

Closing per TicketClosingReasons/UseSupportChannels.