Using Postgres 'serial' in Django (other than primary key)

I have created a custom field for my model BigSerialField

as shown below.

class BigSerialField(models.Field):

description = 'Big Serial of Postgres'

def __init__(self, *args, **kwargs):
    super(BigSerialField, self).__init__(*args, **kwargs)

def deconstruct(self):
    name, path, args, kwargs = super(BigSerialField, self).deconstruct()
    return name, path, args, kwargs

def db_type(self, connection):
    return 'bigserial'

def from_db_value(self, value, expression, connection, context):
    if value is None:
        raise Exception('Big Serial cannot be null!')
    return int(value)

def get_prep_value(self, value):
    return None

def get_db_prep_value(self, value, connection, prepared=False):
    return None

def get_db_prep_save(self, value, connection):
    return self.get_db_prep_value(value, connection)


def to_python(self, value):
    if isinstance(value, int):
        return value

    if value is None:
        raise Exception('Big Serial cannot be null!')

    return int(value)

      

        
        
        
      

    

This raises an error because the ORM is now trying to pass null

to the field on save. A keyword is required instead of Postgres DEFAULT

. However, when I try to send 'default'

instead None

to get_db_prep_save

, Postgres throws an error because it interprets it correctly 'default'

as a string.

So how does Django send the keyword DEFAULT

to Postgres?