On_delete options (CASCADE, PROTECT, SET_NULL) in Django

Introduction

These options tell Django what to do with related data when you delete an item. It helps keep your database clean and consistent.

When deleting a user should also delete all their posts (CASCADE).

When you want to prevent deleting a category if products still use it (PROTECT).

When deleting a parent object but want to keep child objects with a blank reference (SET_NULL).

Syntax

Django

ForeignKey(Model, on_delete=models.CASCADE|models.PROTECT|models.SET_NULL, null=True_if_SET_NULL)

Use null=True with SET_NULL to allow empty references.

These options are used inside Django model fields to control deletion behavior.

Examples

Deletes all posts if the author is deleted.

Django

author = models.ForeignKey(User, on_delete=models.CASCADE)

Prevents deleting a category if products still belong to it.

Django

category = models.ForeignKey(Category, on_delete=models.PROTECT)

Sets profile to null if the related profile is deleted.

Django

profile = models.ForeignKey(Profile, on_delete=models.SET_NULL, null=True)

Sample Program

This example shows three models using different on_delete options to control what happens when related objects are deleted.

Django

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=100)

class Book(models.Model):
    title = models.CharField(max_length=100)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

class Category(models.Model):
    name = models.CharField(max_length=100)

class Product(models.Model):
    name = models.CharField(max_length=100)
    category = models.ForeignKey(Category, on_delete=models.PROTECT)

class Profile(models.Model):
    user_name = models.CharField(max_length=100)

class Comment(models.Model):
    content = models.TextField()
    profile = models.ForeignKey(Profile, on_delete=models.SET_NULL, null=True)

# Explanation:
# Deleting an Author deletes their Books (CASCADE).
# Deleting a Category is blocked if Products use it (PROTECT).
# Deleting a Profile sets Comment.profile to null (SET_NULL).

Important Notes

Always choose on_delete carefully to avoid accidental data loss.

PROTECT raises an error if deletion is blocked, so handle exceptions.

SET_NULL requires null=True on the field.

Summary

CASCADE deletes related objects automatically.

PROTECT stops deletion if related objects exist.

SET_NULL sets the relation to null instead of deleting.