#26650: output_field key-word arg not respected in model query API
-------------------------------------+-------------------------------------
Reporter: freshquiz | Owner: nobody
Type: Bug | Status: new
Component: Database layer | Version: master
(models, ORM) | Keywords: output_field expression
Severity: Normal | query annotate aggregate
Triage Stage: Unreviewed | Has patch: 0
Easy pickings: 0 | UI/UX: 0
-------------------------------------+-------------------------------------
Given the following model:
{{{
import django.db.models
class TempModel(django.db.models.Model):
field1 = django.db.models.DecimalField(max_digits=12,
decimal_places=2)
}}}
...when I run a query like so:
{{{
TempModel.objects.all().aggregate(
my_sum=django.db.models.Sum(
'field1', output_field=django.db.models.DateTimeField()
)
)
}}}
...the result produces a `Decimal` result, no matter what `output_field`
is specified.
I came across this issue by simply wanting to specify the number of
decimal places that would get output in queries such as this.
That led me to find #23941, which shocked me in seeing that the "sane"
behaviour/implementation was replaced with something that breaks
down/invalidates the API.
Then I tried the query above, with all different kinds of `output_field`
values and realised the problem is more systemic and severe, in the sense
that specifying `output_field` is often pointless/futile.
Why can't the implementation actually honour the `output_field` parameter
and construct SQL to cast/coerce fields into the appropriate DB field
types?
The only other means (in which I am aware of) for ensuring that a query
gets compiled to SQL that honours the clients' field type specifications
is to use `.extra()` or `.raw()`, but then that is no longer DB agnostic.
I would have thought it was the job of `output_field` to achieve this in a
DB agnostic manner, utilising the power of the ORM.
--
Ticket URL: <https://code.djangoproject.com/ticket/26650>
Django <https://code.djangoproject.com/>
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