Re: Beginner's question regarding prefetch_related

Hi there,

I think you can use 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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