How to prefetch aggregated @property in Django?

Question

We have two models (simplified versions):

class Contestant(models.Model):
    email = models.EmailField(max_length=255, unique=True)
    # plus some other fi         


        

Answer 1

When your aim is to add aggregated values to each item, you should use annotate, instead of aggregate.

For example (a simple query, no additional methods required):

Contestant.objects.filter(...).annotate(total_points=Sum('points__value'))

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If you really want to put this code out of your query: you can, but a model method is not a right way to do this. Methods on models are for operations on single instances. If you want to do something on a whole QuerySet use an ORM Manager instead.

With a Manager this would look like this:

class TotalPointsManager(models.Manager):
    def get_queryset(self):
        return super(TotalPointsManager, self).get_queryset().annotate(total_points=Sum('points__value'))

class Contestant(models.Model):
    email = models.EmailField(max_length=255, unique=True)

    objects = TotalPointsManager() # You are overriding the default manager!

and then you would construct your query as usual (you can drop prefetch_related):

Contestant.objects.filter(...)

...and total_points field would become "magically" available for every object.