Dan,
The root of the issue here is that you query is asking to retrieve all
fields from
the Parent table and perform an aggregation on the JOIN'ed child table.
That results in the following query
SELECT parent.*, MAX(child.updated_timestamp)
FROM parent
JOIN child ON (child.parent_id = parent.id)
...
Since you SELECT all parent columns Django has no choice but to GROUP BY
parent.* as well because on Oracle GROUP BY must contain all non-aggregate
SELECT clauses. That is not the case on MySQL and PostgreSQL for example.
If you use .values('id') as you described you'll only SELECT parent.id
which will
solve your issue as you've come to discover.
FWIW .only('id') would have worked as well and returned Parent objects
instead
of a dict but all fields accesses would have been deferred.
Also there's a ticket tracking support for aggregation through subquery
that would
have worked for your case[0]
Parent.objects.annotate(
child_updated_timestamp=Child.objects.filter(
parent=OuterRef('pk'),
).aggregate(Max('updated_timestamp'))
)
Cheers,
Simon
[0] https://code.djangoproject.com/ticket/28296
Le lundi 26 novembre 2018 12:32:09 UTC-5, Dan Davis a écrit :
>
> I have a parent model that has a relationship to some data that is
> changing:
>
>
> class Parent(models.Model):
> name = models.CharField(...)
> created_timestamp = models.DateTimeField(auto_now_add=True, null=True)
> updated_timestamp = models.DateTimeField(auto_now=True, null=True)
>
>
> class Child(models.Model):
> parent = models.ForeignKey(Parent, on_delete=models.CASCADE)
> created_timestamp = models.DateTimeField(auto_now_add=True, null=True)
> updated_timestamp = models.DateTimeField(auto_now=True, null=True)
>
>
>
> I am trying to annotate a query with a Max updated_timestamp for the
> children:
>
> Parent.objects.annotate(child_updated_timestamp=models.Max('child__updated_timestamp',
>
> output_field=models.DateTimeField()))
>
>
> It seems like Oracle backend is attempting to GROUP BY every field in the
> child model.
>
> Can anyone tell me whether they've seen anything like this and how to
> constrain the GROUP BY?
>
> Thanks,
>
> -Dan
>
>
>
>
>
>
--
You received this message because you are subscribed to the Google Groups
"Django users" group.
To unsubscribe from this group and stop receiving emails from it, send an email
to django-users+unsubscr...@googlegroups.com.
To post to this group, send email to django-users@googlegroups.com.
Visit this group at https://groups.google.com/group/django-users.
To view this discussion on the web visit
https://groups.google.com/d/msgid/django-users/1e2feec6-bd1f-4ea6-9c85-6f8abd8aaf12%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.
