Django 计算模型中所有对象的复利

Question

Using Python 3.8.1, and Django 3.0.1.

I'm busy with a finance application to track outstanding debits and calculate the interest on the outstanding balances.

I have three models:

• Debtor - which contains the client's personal information as well as the applicable interest rate, compounding period etc.
• DebtorAccount – is created whenever a new client is created. It also keeps a running balance and accrued interest.
• DebtorTransaction – records the type of transaction description, debit/credit, amount etc.

models.py

class Debtor(models.Model):
    name = models.CharField(max_length=200, default="")
    period = models.DecimalField(max_digits=100, decimal_places=2, default=0, null=True)
    interestrate = models.DecimalField(max_digits=100, decimal_places=2, default=0, null=True, blank=True)

    def __str__(self):
        return self.name


@property
def unique_id(self):
    return str(self.pk)


class DebtorAccount(models.Model):
    accountname = models.OneToOneField(Debtor, on_delete=models.CASCADE)
    balance = models.DecimalField(max_digits=100, decimal_places=2, default=0)
    interest = models.DecimalField(max_digits=100, decimal_places=2, default=0)
    rate = models.DecimalField(max_digits=100, decimal_places=2, default=0)
    period = models.DecimalField(max_digits=100, decimal_places=2, default=0)

    def __str__(self):
        return str(self.accountname)


def create_debtoraccount(sender, **kwargs):
    if kwargs['created']:
        debtor_account = DebtorAccount.objects.create(accountname=kwargs['instance'])


post_save.connect(create_debtoraccount, sender=Debtor)


class DebtorTransaction(models.Model):
    CREDIT = 'CREDIT'
    DEBIT = 'DEBIT'
    TRANSACTION_TYPES = (
        ("CREDIT", "Credit"),
        ("DEBIT", "Debit"),
    )

    debtor = models.ForeignKey(DebtorAccount, on_delete=models.CASCADE, blank=True, null=True)
    amount = models.DecimalField(max_digits=100, decimal_places=2, default=0)
    description = models.CharField(max_length=200, blank=True, default="Transaction Description")
    date = models.DateTimeField(auto_now_add=True)
    type = models.CharField(choices=TRANSACTION_TYPES, max_length=6)
    transaction_balance = models.DecimalField(max_digits=100, decimal_places=2, default=0, null=True, blank=True)

    def __str__(self):
        return str(self.debtor)


    @property
    def calculate_interest(user_account):
        user_account = DebtorAccount.objects.all()
        for account in user_account:
            user_interest = (account.balance * (1 + (account.rate / account.period))) - account.balance
            account.save()
            return user_interest

    @property
    def get_transaction_balance(self):
        user_account = DebtorAccount.objects.get(accountname=self.debtor.id)
        if self.type == DebtorTransaction.DEBIT:
            user_balance = user_account.balance + self.amount + self.initiationfee
        else:
            user_balance = user_account.balance - self.amount
        return user_balance


    def save(self, *args, **kwargs):
        self.interest = self.calculate_interest
        self.transaction_balance = self.get_transaction_balance
        super(DebtorTransaction, self).save(*args, **kwargs)


@receiver(post_save, sender=DebtorTransaction, dispatch_uid="transaction log entry")
def update_user_account(sender, instance, **kwargs):
    user_account = DebtorAccount.objects.get(accountname=instance.debtor.id)
    if instance.type == DebtorTransaction.CREDIT:
        user_account.balance = F('balance') - instance.amount
    else:
        user_account.balance = F('balance') + instance.amount

    user_account.save()

Examples of Tables

Clients Table

Transaction Table

I'm using Django Tables2 to display the clients and the transactions. I don't know if it was necessary to have the get_transaction_balance function but it was the only way I was able to have the running balance with the transaction in the Transaction Table.

I now want to calculate the compounded interest on the outstanding balance for each client. I want this to automatically be calculated on the first day of each month for each client. From what I've read Celery is the best way to accomplish this but I haven't used it before and haven't started research just yet.

For now I just want to calculate the interest on each client's outstanding balance. Later on I'll also refine the interest formula to adjust for different compounding periods for different clients.

I'm completely stuck on how to accomplish this. The calculate_interest function is my latest attempt to try and solve this but it doesn't work.

The interest only gets calculated on the outstanding balance of the last client and then applied to the specific instance. It is not calculated for each client individually and saved.

I know the problem is with my for loop and when the save method is overwritten. I just don't know how to solve it.

How do I calculate the interest on the outstanding balance of each client?

Answer 1

Your immediate problem is that you're only calculating the interest for the first instance of DebtorAccount then returning. However, I don't really understand why this is a property that's doing a calculation. This should be a method so the user knows that data is being changed. It's unusual for a property to change data.

@property
def calculate_interest(self): # Use the self argument for properties.
    user_accounts = DebtorAccount.objects.all()
    for account in user_accounts:
        # No need for both the subtraction and the 1 + portion.
        user_interest = account.balance * (account.rate / account.period)
        account.save()
    return # I don't know what you want to return from this

Also, I don't really know why a property on a single instance of DebtorTransaction would perform a change on completely unrelated instances of DebtorAccount. Perhaps you mean to do:

def calculate_interest(self):
    account = self.debtor
    # No need for both the subtraction and the 1 + portion.
    user_interest = account.balance * (account.rate / account.period)
    account.interest = user_interest
    account.save()
    return user_interest

All that said, I'm still not sure your data model is correct due to my lack of understanding of your scenario.