updating account balance with django - django

Question

absolute n00b here, just fiddling around. Trying to make a very simple app to track my personal expenses. I have a class for entering the expenses, a class for the categories and a class for my account balance. Plan is to create en entry in the account balance everytime I create an expense.
How do I update my account balance? I'll have to get fields from the latest entry in expenses to do the math with in my balance class, right?
This is what I have. Any help appreciated.
from django.db import models
from django.utils import timezone
class Category(models.Model):
category = models.CharField(max_length=200,blank=False)
def __str__(self):
return self.category
class Balance(models.Model):
date = models.DateTimeField(default=timezone.now)
previous_balance = ????
transaction = ????
current_balance = previous_balance - transaction
class Expense(models.Model):
date = models.DateTimeField(default=timezone.now)
spender = models.ForeignKey('auth.User')
description = models.CharField(max_length=200)
category = models.ForeignKey(Category,default=1)
ABN = 'ABN'
ING = 'ING'
ACCOUNT_CHOICES = (
(ABN, 'ABN'),
(ING, 'ING'),
)
account = models.CharField(
max_length=30,
choices=ACCOUNT_CHOICES,
default=ABN,
)
amount = models.DecimalField(max_digits=10, decimal_places=2)
def commit(self):
self.commit_date = timezone.now()
self.save()
def __str__(self):
return u"%s. Kosten: %s" % (self.description, self.amount)

Answer 1

If I'm understanding your question correctly, you want to be able to get your current balance after creating Expenses. If so, you can use Django's aggregation:
from django.db.models import Sum
class Balance(models.Model):
date = models.DateTimeField(default=timezone.now)
# Keep the amount you start with
starting_balance = models.IntegerField()
# Get the Sum of all expenses and do some simple subtraction
def get_current_balance(self):
total_expenses = Expense.objects.all().aggregate(Sum('amount'))
return self.starting_balance - total_expenses['amount__sum']
Then in your views, you can do something like:
current_balance = some_balance_instance.get_current_balance()

Answer 2

considered balance change will be trigger by expense record change, you can overwrite save on Expense model. then balance table can be maintain in auto.
import datetime
class Expense(models.Model):
date = models.DateTimeField(default=timezone.now)
spender = models.ForeignKey('auth.User')
description = models.CharField(max_length=200)
category = models.ForeignKey(Category,default=1)
ABN = 'ABN'
ING = 'ING'
ACCOUNT_CHOICES = (
(ABN, 'ABN'),
(ING, 'ING'),
)
account = models.CharField(
max_length=30,
choices=ACCOUNT_CHOICES,
default=ABN,
)
amount = models.DecimalField(max_digits=10, decimal_places=2)
def save(self, *args, **kwargs):
super(Expense, self).save(*args, **kwargs)
last_bal = Balance.objects.order_by('id').last()
Balance.objects.create(date=datetime.datetime.now(), previouse_balance=last_bal.current_balance,
transaction=self, current_balance=last_bal.current_balance + self.amount)