序列化程序中的外键不可json序列化

我试图创建一个序列化程序,指向来自的两个不同对象 Account 和 Currency ,但当它被调用时,我不断收到以下错误: TypeError: Object of type 'Account' is not JSON serializable . 我做错了什么?

class InvoiceSerializer(serializers.ModelSerializer):

    class Meta:
        """Meta class to map serializer's fields with the model fields."""
        model = Invoice
        fields = ('user', 'currency', 'task', 'deposit_amount', 'receive_amount')
        read_only_fields = ('created_at', 'updated_at')

        def create(self, validated_data):
            user = Account.objects.only('id').get(id=validated_data['user'])
            currency = Currency.objects.only('id').get(id=validated_data['currency'])
            return Invoice.objects.create(
                user=user,
                currency=currency,
                task=validated_data['task'],
                deposit_amount=validated_data['deposit_amount'],
                receive_amount=validated_data['receive_amount']
            )

来自请求的数据

{'currency': 2, 'task': 1, 'deposit_amount': 1.005, 'receive_amount': 62793.51, 'user': 23}