import decimal

from django.db import models

class Cash(decimal.Decimal):

    currency = "USD"

class CashField(models.DecimalField):

    def __init__(self, **kwargs):

        kwargs["max_digits"] = 20

        kwargs["decimal_places"] = 2

        super().__init__(**kwargs)

    def from_db_value(self, value, expression, connection):

        cash = Cash(value)

        cash.vendor = connection.vendor

        return cash

class CashModel(models.Model):

    cash = CashField()