👀 Views: 66 💬 Answers: 1 📅 Created: 2025-06-04

django queryset aggregation prefetch_related Python

I'm sure I'm missing something obvious here, but I'm working with Django 3.2 and trying to retrieve a count of related objects through an aggregation while also utilizing `prefetch_related`... However, the count seems to be off when I try to aggregate the number of comments associated with each post in my blog application. Here's the model structure: ```python from django.db import models class Post(models.Model): title = models.CharField(max_length=200) content = models.TextField() class Comment(models.Model): post = models.ForeignKey(Post, related_name='comments', on_delete=models.CASCADE) text = models.TextField() ``` I wrote the following query to fetch all posts along with the number of comments: ```python from django.db.models import Count posts = Post.objects.prefetch_related('comments').annotate(num_comments=Count('comments')) ``` However, when I iterate through the `posts` and print the number of comments for each post, I get unexpected results: ```python for post in posts: print(f'{post.title}: {post.num_comments}') ``` To troubleshoot, I checked the database directly and confirmed that the number of comments for each post is indeed correct. I also verified that the `prefetch_related` is actually fetching the related comments, as they appear in the output when I access them directly. The scenario seems isolated to the aggregation part. I've tried removing the `prefetch_related`, but that only resulted in an N+1 query question. I also experimented with different aggregation methods, but the count remains inconsistent. Could this be an scenario with how Django handles the queryset when both `prefetch_related` and `annotate` are used together? Any insights would be appreciated!