django rest framework get_value with amount=None and currency is not None throws an InvalidOperation exception

[carvincarl]

In the DRF code, when deserializing a JSON object with a value for currency and None for the amount, there is an InvalidOperation exception instantiating the Money object.

Example stack trace:

.tox/django111-py36/lib/python3.6/site-packages/rest_framework/serializers.py:236: in is_valid
    self._validated_data = self.run_validation(self.initial_data)
.tox/django111-py36/lib/python3.6/site-packages/rest_framework/serializers.py:434: in run_validation
    value = self.to_internal_value(data)
.tox/django111-py36/lib/python3.6/site-packages/rest_framework/serializers.py:486: in to_internal_value
    primitive_value = field.get_value(data)
djmoney/contrib/django_rest_framework/fields.py:34: in get_value
    return Money(amount, currency)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = <[AttributeError("'Money' object has no attribute 'amount'") raised in repr()] Money object at 0x10f4cb470>, amount = None
currency = 'USD'

    def __init__(self, amount=Decimal('0.0'), currency=DEFAULT_CURRENCY_CODE):
        if not isinstance(amount, Decimal):
>           amount = Decimal(str(amount))
E           decimal.InvalidOperation: [<class 'decimal.ConversionSyntax'>]

.tox/django111-py36/lib/python3.6/site-packages/moneyed/classes.py:83: InvalidOperation

Here is the method with the problem:

    def get_value(self, data):
        amount = super(MoneyField, self).get_value(data)
        currency = data.get(get_currency_field_name(self.field_name), None)
        if currency:  # Missing a check for amount is None.
            return Money(amount, currency)
        return amount

The fix is to also check that the amount is not None.

        if currency and amount is not None:
            return Money(amount, currency)

Here is a test to expose the error:

    @pytest.mark.parametrize(
        'body, expected', (
            ({'field': '10', 'field_currency': 'EUR'}, Money(10, 'EUR')),
            ({'field': '12.20', 'field_currency': 'GBP'}, Money(12.20, 'GBP')),
            ({'field': '15.15', 'field_currency': 'USD'}, Money(15.15, 'USD')),
            ({'field': None, 'field_currency': 'USD'}, None),  # Add this line for amount is None and currency is not None.
        ),
    )
    def test_post_put_values(self, body, expected):
        serializer = self.get_serializer(NullMoneyFieldModel, data=body)
        serializer.is_valid()
        assert serializer.validated_data['field'] == expected

I will submit a pull request with the test and fix.

Thanks

[ahurracode]

Phew glad to see this..