DemonstratingDbAsCaller.py

#Code Snippet and Logic how Database as caller work. Let's divide into the part to understand logic and the whole concept.

#imported all necessary libaries which is required to perform this activity
import logging
from django.db import transaction
from django.db.models.signals import post_save
from django.dispatch import receiver
from django.apps import apps

#Defined a model for User accounts for performing database operation
class UserAccount(models.Model):
    
    #the UserAccount model has two parameters as username and email. The email field is unique, meaning that no two users can have the same email address.
    username = models.CharField(max_length=255)
    email = models.EmailField(unique=True)

#Defined the signal handler to send a welcome email
@receiver(post_save, sender=UserAccount)

#The signal handler will be sendiing the data as welcome email by creating a new EmailLog entry in the database.
def send_welcome_email(sender, instance, **kwargs):
    # using try and exception to handle a database error while sending the email
    try:
        
        #the with transaction.atomic() block confirms that the creation of the EmailLog object/reference is atomic, meaning that if any errors occur during the creation process, the entire transaction will be rolled back.
        with transaction.atomic():
            # Create a new email log entry
            EmailLog = apps.get_model('myapp', 'EmailLog')
            email_log = EmailLog(user=instance, email_type='welcome')
            email_log.save()
            # Raise an exception to simulate a database error
            raise ValueError('Simulated database error while sending email') #it handles the database error
    except ValueError as e:
        logging.error(f"Error sending welcome email: {e}")

#And the except block catches the ValueError exception and logs an error message using the logging module.

#So it has been shown in the code and logic as below explained final output ------- 

#Django signals, by default, do not run in the same database transaction as the code that calls them.

#This means that if someone creates a new user account, the account gets saved to the database successfully. Even if something goes wrong in the signal handler (like an error while trying to send a welcome email), that error won't rollback the account creation. The account is saved, but the email sending fails separately, without affecting the account creation.

#So, the account is created, but the email doesn’t get sent because of the error.