#31506: ExpressionWrapper() doesn't respect output_field when combining
DateField
and timedelta on PostgreSQL and MySQL.
-------------------------------------+-------------------------------------
Reporter: Matthieu Rigal | Owner: (none)
Type: Bug | Status: new
Component: Database layer | Version: dev
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Comment (by Matthieu Rigal):
Replying to [comment:6 David Sanders]:
> There are a few ways to address this but IMHO I don't think anything
should be done here as this is documented PostgreSQL behaviour [1]
>
> Therefore I'd recommend this ticket be marked `wontfix`.
>
> I think it's the developers responsibility to cast in this situation
instead of using `ExpressionWrapper`:
>
> {{{
> next_segments = StartModel.objects.filter('start__gt':
OuterRef('start')).order_by('start')
> qs = StartModel.objects.annotate(
> end=Cast(
> Subquery(next_segments.values('start')[:1]) -
datetime.timedelta(days=1),
> output_field=DateField(),
> )
> )
> }}}
>
>
> [1] https://www.postgresql.org/docs/current/functions-datetime.html
#OPERATORS-DATETIME-TABLE
Thanks David!
Seeing it from a PostgreSQL perspective, it makes sense, as interval has
the precision of a timestamp!
However, as this is pretty surprising from a Pythonic point-of-view, I
would rather consider extending the documentation to warn about this
potential trap. It should be clear that `timedelta` can only be cast (by
Psycopg2) to an interval and therefore does not allow the use of the `date
+/- integer → date` operation of PostgreSQL.
Besides the example with `Cast` one may add that in case this patterns is
recurrent, it may be worth adding that creating a custom Field `DateDelta`
translating to integer on PostrgeSQL side, could be an option...
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Ticket URL: <https://code.djangoproject.com/ticket/31506#comment:9>
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