Beginner's question regarding prefetch_related

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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