null=True should not be added to CurrencyField (2.1.1 > 3.0)

[flc]

Hi, after upgrading from 2.1.1 to 3.0 makemigrations detects a change for all MoneyField fields declared in a similar way like this:

    min_order_fee = MoneyField(
        _('min_order_fee'),
        max_digits=10,
        decimal_places=2,
        default_currency=settings.DEFAULT_CURRENCY,
        null=True, blank=True,
    )

The created migration:

operations = [
        migrations.AlterField(
            model_name='supplier',
            name='min_order_fee_currency',
            field=djmoney.models.fields.CurrencyField(choices=[('CHF', 'CHF'), ('EUR', 'EUR'), ('GBP', 'GBP'), ('USD', 'USD')], default='USD', editable=False, max_length=3, null=True),
        ),
    ]

The change it detects is that it wants to add null=True to the CurrencyField which is strange because the currency field is a CharField subclass so it should not be nullable. However the MoneyField/DecimalField I'd like to keep as nullable.

[benjaoming]

because the currency field is a CharField subclass so it should not be nullable.

I can't really grasp everything about this issue, but please have a look here: #672 (comment)