python 23 lines · 1 tab

Django select_related and prefetch_related for N+1 query optimization

Priya Sharma Jan 2026

1 tab

from django.db.models import Prefetch
from django.views.generic import ListView
from .models import Post, Comment


class PostListView(ListView):
    model = Post
    template_name = 'blog/post_list.html'
    context_object_name = 'posts'

    def get_queryset(self):
        # Optimize author (ForeignKey) and tags (ManyToMany)
        # Also prefetch recent comments with custom queryset
        recent_comments = Comment.objects.filter(
            approved=True
        ).select_related('author').order_by('-created_at')[:5]

        return Post.objects.select_related(
            'author'
        ).prefetch_related(
            'tags',
            Prefetch('comments', queryset=recent_comments)
        ).filter(published=True)

1 file · python Explain with highlit

The N+1 query problem is Django's most common performance trap. I use select_related for foreign key and one-to-one relationships (performs SQL JOIN), and prefetch_related for many-to-many and reverse foreign keys (separate queries with Python join). The Django Debug Toolbar helps identify these issues early. For complex prefetches, I use Prefetch objects to customize the queryset, like adding ordering or filtering. This dramatically reduces query count from hundreds to single digits, making views fast even with large datasets.

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