How to prevent a field from being "removed" from the model once you turn it into a property in Django?

Question

I have a field that is a property, because it's the average of all reviews:

ratings = models.FloatField(null=True, default=0)
@property
def ratings(self):
    return self.reviews.aggregate(avg_score=Avg('score'))['avg_score']

The problem is that if I do this, then the field "disappears." In fact, if I run makemigrations, it deletes the field. The problem is that I use django-filter, and I want to have the option of filtering by ratings (show me all books that have between a 7 and a 10). If I make it into a property, I can't do it anymore because django-filter can't "detect" the field anymore.

Is there a way for me to still have property and the field at the same time? Can I maybe make a surrogate field that gets updated everytime ratings is changed and make it equal to that?

Answer 1

Well you can not make something both a field and a property at the same time. Furthermore I would advice not to use a field in the first place, since that will introduce data duplication: you save the data in an aggregated form somewhere else. It turns out that synchronizing data, even on the same database, is a much harder problem than it appears to be.

You can annotate(..) [Django-doc] the queryset such that the database will calculate the aggregates automatically per item it retrieves. Such queryset looks like:

from django.db.models import Avg

MyModel.objects.annotate(
    ratings=Avg('reviews__score')
)

If you need this value often, you can do that in the Manager [Django-doc] of the model:

class MyModelManager(models.Manager):

    def get_queryset(self):
        return super().get_queryset().annotate(
            ratings=Avg('reviews__score')
        )

class MyModel(models.Model):
    # … model without a ratings field
    objects = MyModelManager()