Hi there,
I think you can use values()
<https://docs.djangoproject.com/en/2.2/ref/models/querysets/#django.db.models.query.QuerySet.values>
to do a group by. Something along these lines should be able to do it in 3
queries:
from django.models import Max
# aggregate grouping by submission then get those assessment objects
latest_assessments = Assessment.objects.values('id', 'submission', latest=
Max('time_stamp'))
assessments = Assessment.objects.filter(id__in=[a['id'] for a in
latest_assessments])
# map submission_id to the latest assessment
assessments = {a.submission_id: a}
# match latest assessment to submission
submissions = Submission.objects.all()
xs = [(s, assessments.get(s.id, None)) for s in submissions]
On Thursday, 27 June 2019 22:09:29 UTC+3, swimmer wrote:
>
> Dear Django community,
>
> I have a data model that looks like the following:
>
> class Submission(models.Model):
> ...
>
> class Assessment(models.Model):
> submission = models.ForeignKey(
> Submission, on_delete=models.CASCADE, related_name='assessments')
> time_stamp = models.DateTimeField()
>
> And I would like to do something simple such as computing a list of pairs
> of submissions and their most recent assessments:
>
> qs = Submission.objects.prefetch_related('assessments')
> xs = [(s, s.assessments.latest('time_stamp')) for s in qs.all()]
>
> This seems to be inefficient because the assessments table seems to be
> queried for each submission.
> I've read about prefetch_related and tried to use it as above but it
> doesn't seem to have any effect.
> I suppose this is because I try to use 'latest'?
> Is prefetch_related applicable in this case at all or do I need to resort
> to a different technique like a raw SQL query?
>
> Thanks in advance!
>
> Simon
>
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