What are you trying to do?
On Thu, Jun 27, 2019, 2:09 PM swimmer <[email protected]> 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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