"TypeError: <amount> <currency> is not JSON serializable" when serializing a calculated model fields in DRF

[JadSayegh]

I have model which has a calculated MoneyField:

class Account(models.Model):
    deposits = MoneyField(decimal_places=2, max_digits=14, default=Money(0, 'USD'))
    withdrawals = MoneyField(decimal_places=2, max_digits=14, default=Money(0, 'USD'))

    @property
    def net_deposits(self):
        return Money(self.deposits.amount - self.withdrawals.amount, self.deposits.currency)

My serializer looks like this

class AccountSerializer(serializers.ModelSerializer):
    net_deposits = serializers.ReadOnlyField()

    class Meta:
        model = Account
        fields = ('deposits', 'withdrawals', 'net_deposits')

Whenever I try to query a model through this serializer I get the error mentioned in the title, for example:
TypeError: 80 USD is not JSON serializable

This is not an issue when the field is directly on the model (not a calculated field).

I've also tried making it a SerializerMethodField and return net_deposit from the function and am getting the same thing. What's the correct way to get this to work?

[Stranger6667]

Hello @JadSayegh !
You have to explicitly declare MoneyField on AccountSerializer.

class AccountSerializer(serializers.ModelSerializer):
    net_deposits = MoneyField(max_digits=14, decimal_places=2, read_only=True)

    class Meta:
        model = Account
        fields = ('deposits', 'withdrawals', 'net_deposits')

And it will process the money value as usual:

>>> instance = Account.objects.create(deposits=Money(1, 'EUR'), withdrawals=Money(2, 'EUR'))
>>> serializer = AccountSerializer(instance=instance)
>>> serializer.data
{'deposits': '1.00', 'net_deposits': '-1.00', 'withdrawals': '2.00'}

I hope this will help :) If you have any questions - I'd like to hear from you :)

Best regards

[JadSayegh]

Thanks for the quick reply Dmitry (@Stranger6667), my net_deposits are now showing up in my serialized data, but without net_deposits_currency. I tried adding that to the fields but I am getting an error as a result:

django.core.exceptions.ImproperlyConfigured: Field name net_deposits_currency is not valid for model 'Account'.

[Stranger6667]

Since there is no such field on the model you cant use it as an automatic field. But you can do something like this:

class AccountSerializer(serializers.ModelSerializer):
    net_deposits = MoneyField(max_digits=14, decimal_places=2, read_only=True)
    net_deposits_currency = serializers.CharField(source='net_deposits.currency')

    class Meta:
        model = Account
        fields = ('deposits', 'withdrawals', 'net_deposits', 'net_deposits_currency')

And the currency should appear in the serialized data:

>>> instance = Account.objects.create(deposits=Money(1, 'EUR'), withdrawals=Money(2, 'EUR'))
>>> serializer = AccountSerializer(instance=instance)
>>> serializer.data
{'deposits': '1.00', 'net_deposits': '-1.00', 'withdrawals': '2.00', 'net_deposits_currency': 'EUR'}

[JadSayegh]

Awesome, that solves it!

Clarification note for future readers: MoneyField is the one in djmoney.contrib.django_rest_framework and not the one in djmoney.models.fields.

Out of curiosity, if I had a SerializerMethodField that returns a Money object, how would I serialize that? Would the only way be to have the method on the model class as a @property like what I did with net_deposits or is there a way to use a method on the serializer?

[Stranger6667]

Hm, it depends on the context and your personal taste. From some point of view, the object is serialized - you have field and its value (Money instance). Further, if you want to convert the serialized result to JSON you could create your own JSONEncoder (like Django does) and convert this value to some valid JSON output.

Another option - return dictionary from this method instead of Money, but it will be nested structure. In case if you want "flat" structure DRF implies, that you should use separate field for this. But you can unnest this structure later in the serialization process, but it is a bit hacky IMO.

So, there are a plenty of options to do serialization :)

Hope, I've answered your question

Best Regards

[Stranger6667]

I'm closing this issue :) If you have any questions feel free to reopen this or create a new one