django-orm-mastery

Django ORM Mastery

Write efficient, expressive Django ORM queries. The ORM is lazy by default -- QuerySets are not evaluated until iterated. Understanding this is key to avoiding N+1 queries and building performant applications.

When to Use

• User writes Django model queries
• User encounters slow queries or N+1 problems
• User needs complex filtering, aggregation, or annotation
• User asks about select_related vs prefetch_related
• User needs database-level operations without pulling data into Python

Core Patterns

QuerySet Basics and Chaining

QuerySets are lazy -- they build SQL without executing until evaluated. Chain freely.

# Lazy -- no SQL executed yet
qs = (
    Article.objects
    .filter(status="published")
    .exclude(category__name="draft")
    .order_by("-published_at")
)

# SQL executes when you iterate, slice, call list(), len(), bool(), etc.
articles = list(qs[:10])

# Efficient existence check -- use .exists() not len() or bool()
if Article.objects.filter(slug=slug).exists():
    raise ValidationError("Slug already taken")

# Efficient count -- use .count() not len(qs)
total = Article.objects.filter(status="published").count()

# Get or 404
from django.shortcuts import get_object_or_404
article = get_object_or_404(Article, slug=slug, status="published")

select_related (ForeignKey / OneToOne)

Performs a SQL JOIN. Use for ForeignKey and OneToOneField relationships. Fetches related objects in a single query.

# WITHOUT select_related -- N+1 problem
articles = Article.objects.all()
for article in articles:
    print(article.author.name)  # Each access hits the database

# WITH select_related -- single JOIN query
articles = Article.objects.select_related("author", "category").all()
for article in articles:
    print(article.author.name)  # No extra query -- already loaded

# Chain through relationships
comments = (
    Comment.objects
    .select_related("article__author", "article__category")
    .filter(is_approved=True)
)