models.py

from django.db import models

from django.contrib.auth.models import User
from django.db import models, transaction

class Account(models.Model):
    balance = models.IntegerField(default=0)
    user = models.ForeignKey(User)
    
    def get_queryset(self):
        return self.__class__.objects.filter(id=self.id)
     
    @transaction.atomic()
    def deposit(self, amount):
        obj = self.get_queryset().select_for_update().get()
        obj.balance += amount
        obj.save()
    
    @transaction.atomic()
    def withdraw(self, amount):
        obj = self.get_queryset().select_for_update().get()
        if amount > obj.balance:
            raise errors.InsufficientFunds()
        obj.balance -= amount
        obj.save()