Django @property calculating a model field: FieldError: Cannot resolve keyword

Question

I'm following the method used by @Yauhen Yakimovich in this question:

do properties work on django model fields?

To have a model field that is a calculation of a different model.

The Problem:

FieldError: Cannot resolve keyword 'rating' into field. Choices are: _rating

The rating model field inst correctly hidden and overridden by my rating property causing an error when I try to access it.

My model:

class Restaurant(models.Model):    
    ...
    ...
    @property
    def rating(self):
        from django.db.models import Avg
        return Review.objects.filter(restaurant=self.id).aggregate(Avg('rating'))['rating__avg']

Model in Yauhen's answer:

class MyModel(models.Model):
    __foo = models.CharField(max_length = 20, db_column='foo')
    bar = models.CharField(max_length = 20)

    @property
    def foo(self):
        if self.bar:
            return self.bar
        else:
            return self.__foo

    @foo.setter
    def foo(self, value):
        self.__foo = value

Any ideas on how to correctly hid the rating field and define the @property technique?

Answer 1

Solved by using sorted()

I was using a query with order_by() to call rating. order_by() is at the database level and doesnt know about my property. Soultion, use Python to sort instead:

sorted(Restaurant.objects.filter(category=category[0]), key=lambda x: x.rating, reverse=True)[:5]

If you encounter a similar error check through your views for anything that might be calling the property. Properties will no longer work at the datatbase level.

Answer 2

Change this line:

self._rating = Review.objects.filter(restaurant=self.id).aggregate(Avg('rating'))['rating__avg']

into this one:

self._rating = Review.objects.filter(restaurant=self.id).aggregate(Avg('_rating'))['_rating__avg']

(notice change of reference in query from rating and rating__avg to _rating and _rating__avg)