Cheat Sheet for all Django `on_delete` options

Django on_delete Cheat Sheet

Option	What Happens When Referenced Object is Deleted?	Use Case Example
CASCADE	Also delete the object with the ForeignKey	Delete all Comments when a Post is deleted
PROTECT	Raises ProtectedError and prevents deletion	Prevent deleting a User if they’re linked to existing Orders
SET_NULL	Sets the ForeignKey field to NULL	Keep Article, but set author = NULL when Author is deleted
SET_DEFAULT	Sets the ForeignKey field to its default value	If a Product is deleted, fallback to a default Category
SET(...)	Sets the ForeignKey to a custom value or callable	Set to an "archived" user when original User is deleted
DO_NOTHING	Does nothing; deletion might fail due to DB-level constraints	Advanced use with manual control; avoid unless necessary
RESTRICT (Django 3.1+)	Raises error if there are any referencing objects (like PROTECT, but stricter in behavior visibility)	Similar to PROTECT, but enforces DB constraint at the SQL level

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When to Use What?

Goal	Recommended Option
Automatically delete dependents	CASCADE
Prevent deletion if used somewhere	PROTECT or RESTRICT
Allow deletion, but keep dependent object	SET_NULL or SET(...)
Replace with default	SET_DEFAULT
Full manual control	DO_NOTHING

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Example Code Snippets

from django.db import models
from django.conf import settings

class Post(models.Model):
    author = models.ForeignKey(
        settings.AUTH_USER_MODEL,
        on_delete=models.SET_NULL,
        null=True,
        blank=True,
    )

class Comment(models.Model):
    post = models.ForeignKey(Post, on_delete=models.CASCADE)

class Order(models.Model):
    user = models.ForeignKey(settings.AUTH_USER_MODEL, on_delete=models.PROTECT)

class Product(models.Model):
    category = models.ForeignKey(
        "Category",
        on_delete=models.SET_DEFAULT,
        default=1  # assuming category with id=1 is 'Uncategorized'
    )