Django·Django from project to API

Querysets

Filter, annotate and optimise database access instead of looping in Python.

Intermediate 15 min read Lesson 8 of 300 Challenge included

What you will be able to do

Chain filters to build a query.
Use annotate for computed fields.
Use select_related for foreign keys.
Evaluate with list(), len() or iteration only when needed.

In plain English

A queryset is lazy: it builds SQL and runs when evaluated. Use filter, exclude, annotate and aggregate to let the database do the work. select_related and prefetch_related prevent the N+1 query problem.

queries.pyPython

from django.db.models import Count, Avg
from .models import Course

popular = (
    Course.objects.annotate(lesson_count=Count('lessons'), avg_minutes=Avg('lessons__minutes'))
    .filter(lesson_count__gte=5)
    .order_by('-lesson_count')
)

for course in popular:
    print(course.title, course.lesson_count, round(course.avg_minutes or 0, 1))

Your turn

Annotate a count

Medium

Annotate courses with a lesson_count using Count("lessons").

PYTHON · starter

Course.objects.annotate()

Check your understanding before moving on.

Knowledge checkQuestion 1

Which method prevents an N+1 query for a foreign key?

Select an answer to continue

Study tools

Make it yours.

Keep a private note for this lesson, then turn the key ideas into flashcards you can revisit.

Sign in to save notes and flashcards.Both are private to your account and stay with your progress.

Finished reading?

Lock in what you just learned.

Mark this lesson complete to save your place and move one step closer to the certificate.

Next: Static files and deployment

Lesson 8 of 300 · 0% of course