? Common Django Signals

? Quick Overview

In Django, signals provide a powerful mechanism to trigger actions in response to changes in the database. Among the most common signals are pre_save, post_save, and post_delete. They allow you to execute custom logic before or after saving or deleting a model instance.

? Key Concepts

• Signals notify parts of your app when events occur
• Receivers listen and respond to signals
• Used for automation, cleanup, and side effects

? What is a Signal in Django?

Signals are messages sent when certain actions occur in Django. When a model instance is saved or deleted, Django can emit signals that receiver functions can listen to and act upon.

⚙️ pre_save Signal

The pre_save signal is sent just before a model’s save() method is called. It is useful for modifying data before it is stored in the database.

# Set default title before saving the book
from django.db.models.signals import pre_save
from django.dispatch import receiver
from .models import Book

@receiver(pre_save, sender=Book)
def update_book_title(sender, instance, **kwargs):
    if not instance.title:
        instance.title = "Untitled"

⚙️ post_save Signal

The post_save signal runs after a model instance has been saved. It is commonly used for sending notifications or creating related records.

# Send notification after a new book is created
from django.db.models.signals import post_save
from django.dispatch import receiver
from .models import Book, Notification

@receiver(post_save, sender=Book)
def send_book_created_notification(sender, instance, created, **kwargs):
    if created:
        Notification.objects.create(
            message=f"A book titled '{instance.title}' was added."
        )

⚙️ post_delete Signal

The post_delete signal is triggered after a model instance is removed from the database. It is useful for cleaning up related resources.

# Remove associated cover image after deleting the book
from django.db.models.signals import post_delete
from django.dispatch import receiver
from .models import Book

@receiver(post_delete, sender=Book)
def delete_book_cover(sender, instance, **kwargs):
    if instance.cover:
        instance.cover.delete()

? Use Cases

• Auto-fill missing fields
• Send email or system notifications
• Delete unused files from storage