Django Cache Page Fragment Fix

In this tutorial, you'll learn about Django Cache Page Fragment Fix. We cover key concepts, practical examples, and best practices.

The Problem

A page has expensive sections (sidebar analytics, recent comments) that don't change on every request. Caching these fragments saves database queries while keeping dynamic content fresh.

Quick Fix

Wrong — caching the whole page

@cache_page(60 * 15)
def dashboard(request):
    ...

Output: Entire page is cached. All users see the same content. Personalization and CSRF tokens break.

Correct — fragment caching

{% load cache %}

{% cache 900 recent_comments %}
    {% for comment in recent_comments %}
        <div class="comment">{{ comment.text }}</div>
    {% endfor %}
{% endcache %}

Output: Only the comments section is cached for 900 seconds. The rest of the page remains dynamic.

Cache with dynamic keys

{% cache 86400 user_stats request.user.id %}
    <div class="stats">
        Posts: {{ user.post_count }} | Comments: {{ user.comment_count }}
    </div>
{% endcache %}

Output: Each user gets their own cached fragment with a unique key.

Multiple fragments

{% cache 3600 sidebar_categories %}
    {% for cat in categories %}
        <a href="{{ cat.url }}">{{ cat.name }}</a> ({{ cat.post_count }})
    {% endfor %}
{% endcache %}

{% cache 300 recent_posts %}
    {% for post in recent_posts %}
        <h3>{{ post.title }}</h3>
    {% endfor %}
{% endcache %}

Programmatic cache control

from django.core.cache import cache

def update_post(request, post_id):
    post = get_object_or_404(Post, id=post_id)
    post.save()
    # Invalidate the cached sidebar
    cache.delete('sidebar_categories')
    return redirect('post_detail', post_id=post_id)

Conditional caching

{% if user.is_staff %}
    {% cache 60 admin_panel %}
        {% include "admin_panel.html" %}
    {% endcache %}
{% endif %}

Prevention

• Use fragment caching for expensive, semi-static template sections.
• Include user ID or object ID in cache keys for personalized fragments.
• Clear related cache keys when underlying data changes.

Common Mistakes with cache page fragment

1. Forgetting deriving (Show, Eq) on custom data types needed for debugging
2. Placing the wildcard pattern first in case expressions, making all subsequent patterns unreachable
3. Using head and tail instead of pattern matching, causing runtime errors on empty lists

These mistakes appear frequently in real-world DJANGO code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ