#31638: On Mysql/Sqlite, db.models.functions.Now uses wrong timezone
-------------------------------------+-------------------------------------
Reporter: Matthijs | Owner: nobody
Kooijman |
Type: Bug | Status: new
Component: Database | Version: master
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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On Mysql and Sqlite (maybe also Oracle, cannot test that), the
`db.models.functions.Now()` seems to get the current timezone wrong.
- On Mysql, `Now()` / `CURRENT_TIMESTAMP` produces a datetime in the
Mysql session timezone (which defaults to the system timezone).
- On Sqlite, `Now()` / `CURRENT_TIMESTAMP` produces a datetime in UTC.
However, if `USE_TZ` is set, all datetimes stored in the database use the
per-database configured `TIME_ZONE` (defaulting to UTC), resulting in a
potential timezone mismatch. In particular, this means that
`timezone.now()` and `django.db.models.functions.Now()` are not equal.
I realize that Mysql, Sqlite and Oracle are documented to not support
timezones, but that should only mean that they do not explicitly *store*
timezone information along with `datetime` fields and that keeping
timezones consistent relies on external configuration instead. Given that,
I think that `Now()` should still behave as expected on all databases.
=== Potential fix: Setting the session timezone
For MySQL, you can change the session timezone (e.g. connection timezone).
Setting this to the configured database `TIME_ZONE` would make sense and
solve this problem neatly, by letting MySQL know about the timezone used
for data. This should not affect `datetime` fields themselves (which are
still just read and written as-is), but does affect functions like
`CURRENT_TIMESTAMP()`. It also changes the way `timestamp` fields are
interpreted (which are stored as UTC but read and written in the session
timezone), which actually seems like a good thing (this would fix #19312).
This seems like the cleanest approach, but maybe there is a compelling
reason for not setting the session timezone (otherwise, I might think it
would have been set already to fix #19312?).
For details, see https://dev.mysql.com/doc/refman/8.0/en/time-zone-
support.html. In particular:
> The session time zone setting affects display and storage of time values
that are zone-sensitive. This includes the values displayed by functions
such as NOW() or CURTIME(), and values stored in and retrieved from
TIMESTAMP columns. Values for TIMESTAMP columns are converted from the
session time zone to UTC for storage, and from UTC to the session time
zone for retrieval.
> The session time zone setting does not affect values displayed by
functions such as UTC_TIMESTAMP() or values in DATE, TIME, or DATETIME
columns. Nor are values in those data types stored in UTC; the time zone
applies for them only when converting from TIMESTAMP values. If you want
locale-specific arithmetic for DATE, TIME, or DATETIME values, convert
them to UTC, perform the arithmetic, and then convert back.
This fix does not work for Sqlite, which does not seem to have any concept
of session timezone (there is the concept of `'localtime'`, but I'm not
sure how configurable that is).
Also see https://dev.mysql.com/doc/refman/8.0/en/datetime.html that has
some examples on how `datetime` and `timestamp` are interpreted
differently, and also documents that Mysql 8.0.19 and above allow adding a
timezone offset to datetime literals (which is not stored, but used to
convert the literal to the session timezone before storing).
=== Potential fix: converting in `Now()`
Alternatively, this could be fixed just for `Now()`, by converting the
result of `CURRENT_TIMESTAMP()` to the expected timezone (e.g. per-
database `TIME_ZONE`). For Sqlite, this is probably the only option, but
it is also easier, since you can just convert from UTC to whatever
timezone needed (I *think*, I could not find any documentation for
`CURRENT_TIMESTAMP()` for Sqlite). For Mysql, you would need to figure out
the session timezone (unless there is a way to convert timezones *from*
the session timezone implicitly somehow).
The downside of this option is that any other functions (added later or in
raw queries) will still be wrong.
=== Potential fix: Document as limitation
Not really satisfactory, but I guess `django.db.models.functions.Now`
could document this limitation and recommend to use `timezone.now()`
instead (which I think could be used everywhere `Now()` could be used,
except maybe as a default value, and produce nearly identical results).
=== Testcase showing the problem
I've created a draft pullrequest with a testcase to show this problem
here: https://github.com/django/django/pull/12991
With that testcase, on my system (mariadb 5.5.5-10.3.22-MariaDB-
0ubuntu0.19.10.1, CEST/UTC+0200 timezone), I see that:
- With sqlite and `USE_TZ=False`: fails because `Now()` is in UTC and
Django uses its configured `TIME_ZONE` (`America/Chicago`).
- With sqlite and `USE_TZ=True`: works, since `Now()` is in UTC and
Django defaults to UTC.
- With sqlite and `USE_TZ=True` and database
`TIME_ZONE=Europe/Amsterdam`: fails, since `Now()` is in UTC and Django
now uses `Europe/Amsterdam` (for this, I set `TIME_ZONE` in the
`DATABASES` of my settings manually, not sure if I could do that inside a
testcase).
- With Mysql and `USE_TZ=True`: fails, since Mysql uses the system
timezone (UTC+0200) and Django uses UTC.
- With Mysql and `USE_TZ=False`: fails, since Mysql uses the system
timezone (UTC+0200) and Django uses its configured `TIME_ZONE`
(`America/Chicago`).
See the pullrequest for detailed test output, including some debug prints
that help confirm all of the above observations.
=== Conclusion
For now, I'm going to switch to using `timezone.now()`, which I think
should fix my immediate problem, but it would be nice if this could work
as expected out of the box. As for applying a fix, I do not think I am
familiar enough with the db code to really tell what would be the best
solution or implement it (nor will I have the time for it, I'm afraid).
--
Ticket URL: <https://code.djangoproject.com/ticket/31638>
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