Vicente G. Reyes

Posted on May 23 • Originally published at vicentereyes.org

How I Caught and Fixed an N+1 Query in My Django REST API

#python #django #algorithms

Every performant API eventually runs into the same silent killer: the N+1 query problem. It doesn't crash your app. It doesn't throw errors. It just quietly makes every list endpoint slower as your data grows — and it's almost invisible until Sentry flags it in production.

Today, Sentry caught one on my /api/blog-posts/ endpoint. Here's exactly what happened and how I fixed it in three lines of code.

What Is an N+1 Query?

An N+1 query happens when your code fetches a list of N records, then fires an additional query per record to fetch related data — totalling 1 + N database hits instead of a flat 2 or 3.

In Django, this usually happens silently because the ORM is lazy by default. Accessing a related object on a model instance that wasn't eagerly loaded triggers a fresh SELECT on the spot. With 30 blog posts, that's 30 silent queries you never wrote.

The Offending Code

The BlogPostViewSet looked clean on the surface:

class BlogPostViewSet(viewsets.ReadOnlyModelViewSet):
    queryset = BlogPost.objects.all()
    serializer_class = BlogPostSerializer
    lookup_field = "uid"

And the serializer:

class BlogPostSerializer(serializers.ModelSerializer):
    tags = BlogTagSerializer(many=True, read_only=True)
    series = BlogSeriesSerializer(read_only=True)
    ...

Spot the problem? BlogPost has two relations:

series — a ForeignKey to BlogSeries
tags — a ManyToManyField to BlogTag

When DRF serializes a list of 30 posts, it accesses post.series and post.tags on each one. Without eager loading, Django fires two extra queries per post — one to fetch the series, one to fetch the tags. That's 1 + 60 queries for a 30-post list.

The featured action had the same issue:

@action(detail=False, methods=["get"])
def featured(self, request):
    queryset = BlogPost.objects.filter(date_published__isnull=False).order_by(
        "-date_published",
    )[:3]

A fresh BlogPost.objects call with no eager loading.

The Fix

Django gives me two tools for this:

select_related() — for ForeignKey and OneToOne relations. Issues a SQL JOIN and fetches everything in a single query.
prefetch_related() — for ManyToMany and reverse FK relations. Issues a second query and caches the results in Python.

The fix:

class BlogPostViewSet(viewsets.ReadOnlyModelViewSet):
    queryset = BlogPost.objects.select_related("series").prefetch_related("tags")
    serializer_class = BlogPostSerializer
    lookup_field = "uid"

    @action(detail=False, methods=["get"])
    def featured(self, request):
        queryset = (
            BlogPost.objects.select_related("series")
            .prefetch_related("tags")
            .filter(date_published__isnull=False)
            .order_by("-date_published")[:3]
        )
        serializer = self.get_serializer(queryset, many=True)
        return Response(serializer.data)

With 30 posts, the list endpoint now costs 3 queries regardless of dataset size:

SELECT * FROM core_blogpost ...
SELECT * FROM core_blogseries WHERE id IN (...)
SELECT * FROM core_blogtag INNER JOIN core_blogpost_tags WHERE blogpost_id IN (...)

The Bonus Fix

While auditing the blog endpoint, I spotted the same pattern in TestimonialViewSet. Its serializer accesses project.title and project.slug, but the queryset had no select_related:

# Before
queryset = Testimonial.objects.all()

# After
queryset = Testimonial.objects.select_related("project")

One extra line, one less N+1.

How to Spot This in Your Own Code

The pattern is always the same — look for any ViewSet or view where:

The queryset has no select_related or prefetch_related
The serializer accesses a related field (source="relation.field", nested serializers, SerializerMethodField that touches obj.relation)

Tools that help catch this before Sentry does:

django-debug-toolbar — shows query counts per request in the browser
nplusone — raises exceptions in tests when N+1 queries are detected
Sentry Performance — catches it in production with query traces

The best time to catch an N+1 is during code review. Any time you write a nested serializer, ask: does the queryset for this view eagerly load this relation?

Takeaway

The Django ORM's lazy evaluation is a feature, not a bug — but it requires discipline at the queryset layer. A clean-looking viewset with objects.all() is often hiding a query storm one serializer away.

The rule of thumb: every relation accessed in a serializer needs a corresponding select_related or prefetch_related on the queryset. Make it a checklist item on every PR that touches a ViewSet.

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