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Django QuerySets

A QuerySet is a database query description until something evaluates it. Chaining filters usually builds that description rather than immediately fetching rows.

How it works

Iteration, list, len and Boolean testing can cause evaluation. exists and count answer narrower questions, although their usefulness depends on whether you will need the rows immediately afterwards. A cached evaluated queryset can behave differently from a new equivalent query. Measure the actual work when performance matters instead of assuming that fewer Python lines mean fewer database queries.

Run in the supplied local Django project

This lesson uses Django 5.2.18 LTS with Python 3.12. Download the source downloader and run node download-project.mjs http://127.0.0.1:8000 with Node 22 while previewing locally. It creates a new bhylu-django-lab folder and refuses to overwrite an existing one. After deployment, the default base is bhylu.com; until then use your local preview URL.

Inside that new folder, create a virtual environment, install requirements.txt and run the complete lesson file shown below. Every lesson creates a fresh in-memory SQLite database and temporary upload storage. It does not edit the interactive lab’s db.sqlite3 or Bhylu’s database. The visible listing below is the lesson’s core code; its complete downloaded file includes the shared bootstrap.

Django does not run in Bhylu’s current browser playground. No Apache, MySQL or global PHP configuration change is needed. See the setup lesson for the full project and commands.

Example

Run .venv\Scripts\python.exe lessons\querysets.py from the downloaded project root. Download the complete lesson file; keep it under lessons/. The project’s shared models, forms, views and fixture are prerequisites, not a hidden online runtime.

lessons/querysets.py — core code

from django.db import connection
from django.test.utils import CaptureQueriesContext
with CaptureQueriesContext(connection) as queries:
    rows = Workshop.objects.filter(published=True)
    print(len(queries))
    titles = list(rows.values_list("title", flat=True))
    print(len(queries))
print(titles)

Expected result

0
1
['Lamp care', 'Bike punctures']

Constructing the query runs no SQL, while materializing the selected titles runs one query. The default model ordering makes this fixture’s output deterministic. The result is a Python list and no longer a lazy queryset.

Use Django’s versioned official documentation for exact API contracts and the deployment checklist before a separate release.

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