[Python] How to use add(), set() and clear() methods of ManyRelatedManager in Django...

I have models as follows,

Class Bank(models.Model):
customers = models.ManyToManyField(Customer,'banks', through='bank_customer')

Class Customer(models.Model):
name = models.CharField(max_length=50)

Class Bank_customer(models.Model):
customer = models.ForeignKey(Customer,on_delete=models.CASCADE)
bank = models.ForeignKey(Bank,on_delete=models.CASCADE)
city = models.ForeignKey(City,on_delete=models.CASCADE)

Class City(models.Model):
name = models.CharField(max_length=100)


How do I add Customer objects to Bank? The following does not work

bank.customers.add(customer)


Here bank and customer are saved instances of their classes. Doing this violates not_null constraint for city ForeignKey in Bank_customer table.

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