prefetch_related problem

import datetime

from django.db.models import Q, Avg
from django.http import Http404
from django.shortcuts import render, get_object_or_404

from . import models


def good_reviews(request):
    reviews = models.Review.objects.filter(rating__gte=3)
    return render(request, 'products/reviews.html', {'reviews': reviews})


def recent_reviews(request):
    six_months_ago = datetime.datetime.today() - datetime.timedelta(days=180)
    reviews = models.Review.objects.exclude(created_at__lt=six_months_ago)
    return render(request, 'products/reviews.html', {'reviews': reviews})


def product_detail(request, pk):
    try:
        product = models.Product.objects.prefetch_related('review_set').get(pk=pk)
    except models.Course.DoesNotExist:
        raise Http404
    else:
        # product = get_object_or_404(models.Product, pk=pk)
        four_weeks_ago = datetime.datetime.today() - datetime.timedelta(weeks=4)
        reviews = product.review_set.filter(Q(rating__gte=8)|Q(created_at__gte=four_weeks_ago)).order_by('-created_at').all()
    return render(request, 'products/product_detail.html', {'product': product, 'reviews': reviews})


def product_list(request):
    products = models.Product.objects.annotate(avg_rating=Avg('review__rating')).all()
    return render(request, 'products/product_list.html', {'products': products})