[Fixed]-Update Django Model on Create

from django.db.models.signals import post_save
from django.dispatch import receiver

@receiver(post_save, sender=Transaction)
def change_account_amt(sender, instance, created, **kwargs):
    if created:
        # instance is a Transaction created object ...
        acc_obj = Account.objects.get(...)
        acc_obj.amt = ...
        acc_obj.save()

You can do that easily via a post_save signal. It is sent at the end of the save() method.

from django.db.models.signals import post_save


class Transaction(models.Model):
    ....

class Account(models.Model):
    ... 

# method for updating account amount
def update_account_amount(sender, **kwargs):
    instance = kwargs['instance']
    created = kwargs['created']
    raw = kwargs['raw']
    if created and not raw:
        account = Account.objects.get(...) # get the account object
        account.amt -= instance.amt  # update the amount
        account.save() # save the account object

# register the signal
post_save.connect(update_account_amount, sender=Transaction)