fix(ModelWithNullableCurrency): error was raised when trying to do a select

[woile]

Hello everyone, I found this weird error, I also added a patch, but I would like someone to take a first look.

For the error observe the first test I've added:

def test_no_fail_all(self):
    money = Money(100, 'EUR')
    instance = ModelWithNullableCurrency(money=money)
    instance.save()
    r = ModelWithNullableCurrency.objects.all()
    assert r[0].money == money

After doing that an error like this appears:

~/projects/tmp/venv/lib/python3.6/site-packages/django/db/models/base.py in __init__(self, *args, **kwargs)
    500                 if val is _DEFERRED:
    501                     continue
--> 502                 _setattr(self, field.attname, val)
    503         else:
    504             # Slower, kwargs-ready version.

~/projects/tmp/venv/lib/python3.6/site-packages/djmoney/models/fields.py in __set__(self, obj, value)
    115         if value is not None and self.field._currency_field.null and not isinstance(value, MONEY_CLASSES):
    116             # For nullable fields we need either both NULL amount and currency or both NOT NULL
--> 117             raise ValueError('Missing currency value')
    118         if isinstance(value, BaseExpression):
    119             if isinstance(value, Value):

ValueError: Missing currency value

Steps to reproduce:

1. Create instance an assigned a Money value
2. Save instance
3. Do a Model.objects.all()
4. Observe "ValueError: Missing currency value"

[codecov-io]

Codecov Report

Merging #427 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #427   +/-   ##
=======================================
  Coverage   97.55%   97.55%           
=======================================
  Files          29       29           
  Lines         900      900           
  Branches      153      153           
=======================================
  Hits          878      878           
  Misses         15       15           
  Partials        7        7

Impacted Files	Coverage Δ
djmoney/models/fields.py	98.97% <100%> (ø)	⬆️

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[Stranger6667]

Hello @woile !
Thank you so much for reporting the issue :) Your efforts are very appreciated!

Could you, please, rewrite the tests in the following way, to be more in line with the rest of the code?

    def test_query_not_null(self):
        money = Money(100, 'EUR')
        ModelWithNullableCurrency.objects.create(money=money)
        instance = ModelWithNullableCurrency.objects.get()
        assert instance.money == money

    def test_query_null(self):
        ModelWithNullableCurrency.objects.create()
        instance = ModelWithNullableCurrency.objects.get()
        assert instance.money is None
        assert instance.money_currency is None

And add an entry to the changelog?

Thank you

[woile]

Done!

[Stranger6667]

Thank you! :)