Accounts credit tracking for Django

Accounts can be used to implement a variety of interesting components, including:

• Giftcards
• Web accounts
• Loyalty schemes

Basically anything that involves tracking the movement of funds within a closed system.

This package uses double-entry bookkeeping where every transaction is recorded twice (once for the source and once for the destination). This ensures the books always balance and there is full audit trail of all transactional activity.

If your project manages money, you should be using a library like this. Your finance people will thank you.

Features

• An account has a credit limit which defaults to zero. Accounts can be set up with no credit limit so that they are a 'source' of money within the system. At least one account must be set up without a credit limit in order for money to move around the system.

• Accounts can have:

  • No users assigned
  • A single "primary" user - this is the most common case
  • A set of users assigned

• A user can have multiple accounts

• An account can have a start and end date to allow its usage in a limited time window

• Accounts can be categorised

Installation

Install with:

pip install django-account-balances

Then add accounts to your INSTALLED_APPS and run

./manage migrate accounts

Finally, set-up a daily cronjob that calls:

./manage.py close_expired_accounts

to close any expired accounts and transfer their funds to the 'expired' account.

API

Create account instances using the manager:

from decimal import Decimal
import datetime

from django.contrib.auth.models import User

from accounts import models

anonymous_account = models.Account.objects.create()

barry = User.objects.get(username="barry")
user_account = models.Account.objects.create(primary_user=barry)

no_credit_limit_account = models.Account.objects.create(credit_limit=None)
credit_limit_account = models.Account.objects.create(credit_limit=Decimal('1000.00'))

today = datetime.date.today()
next_week = today + datetime.timedelta(days=7)
date_limited_account = models.Account.objects.create(
	start_date=today, end_date=next_week)

Transfer funds using the facade:

from accounts import facade

staff_member = User.objects.get(username="staff")
trans = facade.transfer(source=no_credit_limit_account,
						destination=user_account,
						amount=Decimal('10.00'),
						user=staff_member)

Reverse transfers:

facade.reverse(trans, user=staff_member, 
				description="Just an example")

If the proposed transfer is invalid, an exception will be raised. All exceptions are subclasses of accounts.exceptions.AccountException. Your client code should look for exceptions of this type and handle them appropriately.

Client code should only use the accounts.models.Budget class and the two functions from accounts.facade - nothing else should be required.

Error handling

Note that the transfer operation is wrapped in its own database transaction to ensure that only complete transfers are written out. When using Django's transaction middleware, you need to be careful. If you have an unhandled exception, then account transfers will still be committed even though nothing else will be. To handle this, you need to make sure that, if an exception occurs during your post-payment code, then you roll-back any transfers.

Here's a toy example:

from accounts import facade

def submit(self, order_total):
	# Take payment first
	transfer = facade.transfer(self.get_user_account(),
							   self.get_merchant_account(),
							   order_total)
	# Create order models
	try:
		self.place_order()
	except Exception, e:
		# Something went wrong placing the order.  Roll-back the previous
		# transfer
		facade.reverse(transfer)

In this situation, you'll end up with two transfers being created but no order. While this isn't ideal, it's the best way of handling exceptions that occur during order placement.

Multi-transfer payments

Projects will often allow users to have multiple accounts and pay for an order using more than one. This will involve several transfers and needs some careful handling in your application code.

It normally makes sense to write your own wrapper around the accounts API to encapsulate your business logic and error handling. Here's an example:

from decimal import Decimal as D
from accounts import models, exceptions, facade


def redeem(order_number, user, amount):
    # Get user's non-empty accounts ordered with the first to expire first
    accounts = models.Account.active.filter(
        user=user, balance__gt=0).order_by('end_date')

    # Build up a list of potential transfers that cover the requested amount
    transfers = []
    amount_to_allocate = amount
    for account in accounts:
        to_transfer = min(account.balance, amount_to_allocate)
        transfers.append((account, to_transfer))
        amount_to_allocate -= to_transfer
        if amount_to_allocate == D('0.00'):
            break
    if amount_to_allocate > D('0.00'):
        raise exceptions.InsufficientFunds()

    # Execute transfers to some 'Sales' account
    destination = models.Account.objects.get(name="Sales")
    completed_transfers = []
    try:
        for account, amount in transfers:
            transfer = facade.transfer(
                account, destination, amount, user=user,
                description="Order %s" % order_number)
            completed_transfers.append(transfer)
    except exceptions.AccountException, transfer_exc:
        # Something went wrong with one of the transfers (possibly a race condition).
        # We try and roll back all completed ones to get us back to a clean state.
        try:
            for transfer in completed_transfers:
                facade.reverse(transfer)
        except Exception, reverse_exc:
            # Uh oh: No man's land.  We could be left with a partial redemption. This will
            # require an admin to intervene.  Make sure your logger mails admins on error.
            logger.error("Order %s, transfers failed (%s) and reverse failed (%s)",
                         order_number, transfer_exc, reverse_exc)
            logger.exception(reverse_exc)

        # Raise an exception so that your client code can inform the user appropriately.
        raise RedemptionFailed()
    else:
        # All transfers completed ok
        return completed_transfers

As you can see, there is some careful handling of the scenario where not all transfers can be executed.

    try:
        transfers = api.redeem(order_number, user, total_incl_tax)
    except Exception:
        # Inform user of failed payment
    else:
        for transfer in transfers:
            source_type, __ = SourceType.objects.get_or_create(name="Accounts")
            source = Source(
                source_type=source_type,
                amount_allocated=transfer.amount,
                amount_debited=transfer.amount,
                reference=transfer.reference)
            self.add_payment_source(source)

Core accounts and account types

A post-syncdb signal will create the common structure for account types and accounts. Some names can be controlled with settings, as indicated in parentheses.

• Assets

  • Sales

    • Redemptions (ACCOUNTS_REDEMPTIONS_NAME) - where money is transferred to when an account is used to pay for something.
    • Lapsed (ACCOUNTS_LAPSED_NAME) - where money is transferred to when an account expires. This is done by the 'close_expired_accounts' management command. The name of this account can be set using the ACCOUNTS_LAPSED_NAME.

  • Cash

    • "Bank" (ACCOUNTS_BANK_NAME) - the source account for creating new accounts that are paid for by the customer (eg a giftcard). This account will not have a credit limit and will normally have a negative balance as money is only transferred out.

  • Unpaid - This contains accounts that are used as sources for other accounts but aren't paid for by the customer. For instance, you might allow admins to create new accounts in the dashboard. An account of this type will be the source account for the initial transfer.

• Liabilities

  • Deferred income - This contains customer accounts/giftcards. You may want to create additional account types within this type to categorise accounts.

Example transactions

Consider the following accounts and account types:

• Assets
  • Sales
    • Redemptions
    • Lapsed
  • Cash
    • Bank
  • Unpaid
    • Merchant funded
• Liabilities
  • Deferred income

Note that all accounts start with a balance of 0 and the sum of all balances will always be zero.

A customer purchases a £50 giftcard

• A new account is created of type 'Deferred income' with an end date
• £50 is transferred from the Bank to this new account

A customer pays for a £30 order using their £50 giftcard

• £30 is transferred from the giftcard account to the redemptions account

The customer's giftcard expires with £20 still on it

• £20 is transferred from the giftcard account to the lapsed account

The customer phones up to complain and a staff member creates a new giftcard for £20

• A new account is created of type 'Deferred income'
• £20 is transferred from the "Merchant funded" account to this new account

Settings

There are settings to control the naming and initial unpaid and deferred income account types:

• ACCOUNTS_MIN_INITIAL_VALUE The minimum value that can be used to create an account (or for a top-up)

• ACCOUNTS_MAX_INITIAL_VALUE The maximum value that can be transferred to an account.

Contributing

Fork repo, set-up virtualenv and run:

make install

Run tests with:

./runtests.py

History

This project used to depend on the e-commerce framework Oscar in a number of places. You may be interested in the upstream repos if you want Oscar integration. This repo was forked off for people who want account balances without Oscar integration.