Group by and Aggregate with nested Field

Question

I want to group by with nested serializer field and compute some aggregate function on other fields.

My Models Classes:

class Country(models.Model):
    code = models.CharField(max_length=5, unique=True)
    name = models.CharField(max_length=50)

class Trade(models.Model):
    country = models.ForeignKey(
        Country, null=True, blank=True, on_delete=models.SET_NULL)
    date = models.DateField(auto_now=False, auto_now_add=False)
    exports = models.DecimalField(max_digits=15, decimal_places=2, default=0)
    imports = models.DecimalField(max_digits=15, decimal_places=2, default=0)

My Serializer Classes:

class CountrySerializers(serializers.ModelSerializer):
    class Meta:
        model = Country
        fields = '__all__'


class TradeAggregateSerializers(serializers.ModelSerializer):
    country = CountrySerializers(read_only=True)
    value = serializers.DecimalField(max_digits=10, decimal_places=2)

    class Meta:
        model = Trade
        fields = ('country','value')

I want to send import or export as query parameters and apply aggregate (avg) over it shown by distinct countries

My View Class:

class TradeAggragateViewSet(viewsets.ModelViewSet):
    queryset = Trade.objects.all()
    serializer_class = TradeAggregateSerializers

    def get_queryset(self):
        import_or_export = self.request.GET.get('data_type')
        queryset = self.queryset.values('country').annotate(value = models.Avg(import_or_export))
    return queryset

I want to get the data in format like:

[
    {
        country:{
            id: ...,
            code: ...,
            name: ...,
        },
        value:...
    },
    ...
]

But having an error on country serializer

AttributeError: Got AttributeError when attempting to get a value for field `code` on serializer `CountrySerializers`. 
The serializer field might be named incorrectly and not match any attribute or key on the `int` instance. 
Original exception text was: 'int' object has no attribute 'code'.

Answer 1

I have found the solution. Actually to_representation in serializer class got only the id of country not its object so i override to_representation as:

class TradeAggregateSerializers(serializers.ModelSerializer):
...
...
    def to_representation(self, instance):
        #instance['country'] = some id, not object
        #getting actual object
        country =  Country.objects.get(id=instance['country'])
        instance['country'] = country
        data = super(TradeAggregateSerializers, self).to_representation(instance)
        return data