#37014: SQLite emulation of db_default involving Extract/Trunc might
unexpectedly
apply overridden timezone for inserted values
-------------------------------------+-------------------------------------
Reporter: Jacob Walls | Type: Bug
Status: new | Component: Database
| layer (models, ORM)
Version: 6.0 | Severity: Normal
Keywords: tzinfo, TIME_ZONE, | Triage Stage:
Extract, Trunc | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
As developed in this [https://forum.djangoproject.com/t/extract-isnt-safe-
to-use-in-db-default-on-sqlite-if-time-zone-changes-at-some-point/44781
forum post], if
- `USE_TZ = True`
- a model field uses `Extract` or `Trunc` in a `db_default` expression
- a test case uses `override_settings()` to test timezone behavior
... then in scenarios where Django must emulate the database default for
SQLite in Python, the value will be transformed according to the
overridden timezone setting, but this is an improper emulation, since the
database's default cannot be overridden by merely flipping a Django
setting.
The scenarios where Django must emulate the db default for SQLite vary
depending on edges like:
- whether any field's value has been set
- whether the field with Extract/Trunc is last
- possibly: whether create() vs. bulk_create() is called
----
There are two ways to fix this as described on forum:
- introduce a heuristic (Simon: `if ddl_compiler_heuristic():`) to switch
on whether to use the project default timezone or the test's overridden
one
- implement `supports_default_keyword_in_insert` on SQLite by emulating it
in its insert compiler
Having only thought about it today, the second approach seems like it
would be simpler to implement and potentially pay broader dividends for
other use cases.
Related ticket: #37013
--
Ticket URL: <https://code.djangoproject.com/ticket/37014>
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