Models for double-book-keeping accounting for managed budgets.

models.py

from django.db import models


class Ledger(models.Model):
    # There can only be one 'Labour' and 'Shop' ledger but lots of 'Budget'
    # ledgers.
    LABOUR, BUDGET, SHOP = 'Labour', 'Budget', 'Shop'
    LEDGER_CHOICES = (
        (LABOUR, LABOUR),
        (BUDGET, BUDGET),
        (SHOP, SHOP),
    )
    type = models.CharField(max_length=128, choices=LEDGER_CHOICES)
    name = models.CharField(max_length=128, unique=True)


class Transaction(models.Model):
    # Every transfer of money should create two rows in this table.
    # (a) the debit from the source ledger
    # (b) the credit to the destination ledger
    ledger = models.CharField(max_length=128)

    debit_amount = models.DecimalField(decimal_places=2, max_digits=12)
    credit_amount = models.DecimalField(decimal_places=2, max_digits=12)

    date_created = models.DateTimeField(auto_now_add=True)


class Budget(models.Model):
    name = models.CharField(max_length=128, unique=True)
    ledger = models.OneToOneField(Ledger, related_name='transactions')

    # This is entered at checkout to pay using budget
    code = models.CharField(max_length=128, unique=True)

    # Audit information
    initial_balance = models.DecimalField(decimal_places=2, max_digits=12)
    current_balance = models.DecimalField(decimal_places=2, max_digits=12)

    expiry_date = models.DateField()

    users = models.ManyToManyField('auth.User')
    products = models.ManyToManyField('catalogue.Product')

    date_created = models.DateTimeField(auto_now_add=True)
    date_last_transation = models.DateTimeField(null=True)