Django Model Relationships

How it works

A workshop belongs to one venue, while a venue can have many workshops through its related name. PROTECT prevents deleting a venue still used by a workshop, preserving the meaning of those records. CASCADE is appropriate only when dependent records should truly disappear with their parent. These choices are application decisions; do not use cascading deletion merely to make a demonstration easier.

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\relationships.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.

from django.db.models.deletion import ProtectedError
venue = Venue.objects.get(code="river-hall")
print(venue.workshops.count())
try:
    venue.delete()
except ProtectedError:
    print("Protected venue")
print(Venue.objects.filter(pk=venue.pk).exists())

Expected result

2
Protected venue
True

Two workshops refer to River Hall, so the deletion is rejected and the venue remains. The exception is an expected boundary, not a reason to remove the protective relationship. A real administration flow should explain which dependencies need attention.