Serializer with `extra_fields` option: add fields from the query arguments

serializers.py

class DefinitionSerializer(UserRelatedSerializer):
    extra_fields = ('unread_notifications_count',)

    def __init__(self, *args, **kwargs):
        """
        Redefine __init__ to add extra_fiels feature.
        You must add the field name to self.extra_fields, and add a method
        `get_field_name` since internally this serialzier creates a
        SerializerMethodField for each extra_field that is also in the
        'extra_fields' query_param.
        """
        res = super(APIV2ModelSerializer, self).__init__(*args, **kwargs)

        request = self.context.get('request')
        if self.extra_fields and request:
            extra_fields = request.query_params.get('extra_fields', '').split(',')
            for field in extra_fields:
                if field in self.extra_fields:
                    self.fields[field] = serializers.SerializerMethodField()
                    
        return res
        
    def get_unread_notifications_count(self, obj):
        return obj.notifications.filter(unread=True).count()

    class Meta:
        model = models.Definition
        fields = '__all__'