[Django]-Create a nested serializer with Django Rest Framework, but without primary keys as identity

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The way I solved this required customising the RegionValueSerializer, intercepting the conversion from native python data types to the field.

class RegionValueSerializer(serializers.ModelSerializer):

    def field_from_native(self, data, files, field_name, into):
        # We need to check all the data items, and ensure they
        # are matched to an existing primary id if they already
        # present

        # Returns nothing because this method mutates 'into'
        super(RegionValueSerializer, self).field_from_native(data, files, field_name, into)

        map_answer = self.parent.object
        new_into = []
        for rv in into.get('regionvalue_set'):
            if rv.id is None:
                try:
                    existing_rv = RegionValue.objects.get(answer=map_answer, region_id=rv.region_id)
                    existing_rv.value = rv.value
                    rv = existing_rv
                except RegionValue.DoesNotExist:
                    pass
            new_into.append(rv)
        into['regionvalue_set'] = new_into

    def get_identity(self, data):
        try:
            # Technically identity is defined by region_id AND self.parent.object.id,
            # but we assume that RegionValueSerializer will only ever be used as a
            # field that is part of MapAnswerSerializer.
            return data.get('region_id', None)
        except AttributeError:
            return None

Caveats: Note that some of these methods are not really discussed in rest_framework‘s docs, so I’m not sure how stable this will be. Also, this solution hits the database more than really necessary (the lookup for existing values is duplicating lookups that occur in the parent Serializer).

👤ferrouswheel

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