models.py

from django.db import models
from django.db.models import Avg


class Blog(models.Model):
    name = models.CharField(max_length=100)
    rating = models.DecimalField(max_digits=5, decimal_places=2, null=True)

    def __str__(self):
        return self.name

    def calc_average(self):
        return self.entry_set.aggregate(res=Avg('rating'))['res']

class Entry(models.Model):
    blog = models.ForeignKey(Blog, on_delete=models.CASCADE)
    headline = models.CharField(max_length=255)
    rating = models.IntegerField(default=5)

    def __str__(self):
        return self.headline

tests.py

from django.db.models import Avg, OuterRef, Subquery
from django.test import TestCase

from .models import Blog, Entry


class BlogTestCase(TestCase):
    def setUp(self):
        # preparing some fake data

        for blog_num in range(0, 3):
            blog = Blog.objects.create(name='{}'.format(blog_num))

            for entry_num in range(0, 4):
                headline = 'Entry {} for blog {}'.format(entry_num, blog_num)
                Entry.objects.create(blog=blog, headline=headline, rating=blog_num * 10)

    def test_calc_average(self):
        blog = Blog.objects.get(name='1')
        self.assertEqual(blog.calc_average(), 10)
        blog = Blog.objects.get(name='2')
        self.assertEqual(blog.calc_average(), 20)

    def test_avg(self):
        Blog.objects.update(
            rating=Subquery(
                Blog.objects.filter(
                    id=OuterRef('id')
                ).annotate(
                    avg_rating=Avg('entry__rating')
                ).values('avg_rating')
            )
        )

        blog = Blog.objects.get(name='1')
        self.assertEqual(blog.calc_average(), 10)
        blog = Blog.objects.get(name='2')
        self.assertEqual(blog.calc_average(), 20)