Code snippet to demonstrate how a Django model can be made immutable on certain condition. Hint : Override the save() method for the model in question.

models.py

from django.db import models
from django.utils import timezone


class Bank(models.Model):
    bank_ifsc = models.CharField(max_length=200, blank=False, primary_key=True)
    bank_name = models.CharField(max_length=200, blank=False)
    bank_addr = models.CharField(max_length=400, blank=True)

    def __str__(self):
        return self.bank_name

    def save(self, *args, **kwargs):
        if len(Bank.objects.filter(bank_name=self.bank_name)) >= 2:
            print("Branch limit for bank ", self.bank_name, " exceeded")
        else:
            super().save(args, kwargs)
        return


class Banker(models.Model):
    banker_uid = models.CharField(max_length=200, blank=False, primary_key=True)
    banker_name = models.CharField(max_length=200, blank=False)
    banker_age = models.IntegerField(blank=True, default=18)
    banker_acct = models.ForeignKey(Bank, on_delete=models.CASCADE)

    def __str__(self):
        return "%s %s" % (self.banker_uid, self.banker_name)


class Transactions(models.Model):
    transaction_datetime = models.DateTimeField(default=timezone.now)
    transaction_bank = models.ForeignKey(Bank, on_delete=models.CASCADE)
    transaction_banker = models.ForeignKey(Banker, on_delete=models.CASCADE)

    def __str__(self):
        return "%s : (%s, %s)" % (self.transaction_datetime,
                                  self.transaction_banker,
                                  self.transaction_bank)

    def save(self, *args, **kwargs):
        try:
            Transactions.objects.get(transaction_datetime=self.transaction_datetime,
                                     transaction_bank=self.transaction_bank,
                                     transaction_banker=self.transaction_banker)
            print("Existing record. Not action intiated")
        except Transactions.DoesNotExist:
            print("New record. Saving to the database")
            super().save(args, kwargs)
        return