pre_save and post_save signals in Django - Mini Project: Build & Apply

Using Django pre_save and post_save Signals

📖 Scenario: You are building a simple Django app to manage books in a library. You want to automatically update a timestamp before saving a book and log a message after saving it.

🎯 Goal: Create a Django model for Book and use pre_save and post_save signals to update a field and print a message.

📋 What You'll Learn

Create a Book model with fields title (string) and last_updated (DateTimeField).

Create a pre_save signal handler that updates last_updated to the current time before saving.

Create a post_save signal handler that prints 'Book saved!' after saving.

Connect the signal handlers properly to the Book model.

💡 Why This Matters

🌍 Real World

Automatically updating timestamps and triggering actions on model changes is common in web apps for data integrity and notifications.

💼 Career

Understanding Django signals is important for backend developers to handle side effects and keep code clean and modular.

1

Create the Book model

Create a Django model called Book with a title field as models.CharField(max_length=100) and a last_updated field as models.DateTimeField(null=True, blank=True).

Django

# Define the Book model with title and last_updated fields
# Your code here

Need a hint?

Use models.CharField for the title and models.DateTimeField for last_updated with null=True and blank=True.

2

Create a pre_save signal handler

Import pre_save from django.db.models.signals and receiver from django.dispatch. Create a function called update_last_updated that takes sender, instance, and **kwargs. Inside it, set instance.last_updated to timezone.now(). Use @receiver(pre_save, sender=Book) to connect it.

Django

from django.db import models
from django.db.models.signals import pre_save
from django.dispatch import receiver
from django.utils import timezone

class Book(models.Model):
    title = models.CharField(max_length=100)
    last_updated = models.DateTimeField(null=True, blank=True)

# Create pre_save signal handler here
# Your code here

Need a hint?

Use @receiver(pre_save, sender=Book) decorator and set instance.last_updated = timezone.now().

3

Create a post_save signal handler

Import post_save from django.db.models.signals. Create a function called notify_book_saved that takes sender, instance, created, and **kwargs. Inside it, print the string 'Book saved!'. Use @receiver(post_save, sender=Book) to connect it.

Django

from django.db.models.signals import post_save

# Create post_save signal handler here
# Your code here

Need a hint?

Use @receiver(post_save, sender=Book) and print 'Book saved!' inside the function.

4

Connect signals in apps.py

In your app's apps.py, import AppConfig from django.apps. Create a class called LibraryConfig inheriting from AppConfig with name = 'library'. Override the ready() method to import the models module with import library.models.

Django

# Define LibraryConfig class inheriting from AppConfig
# Your code here

Need a hint?

Override ready() in LibraryConfig to import library.models so signals connect on app start.