class PurchaseSerializer(serializers.HyperlinkedModelSerializer):

    def get_fields(self, *args, **kwargs):
        fields = super(PurchaseSerializer, self).get_fields(*args, **kwargs)
fields['purchaser'].queryset = permitted_objects(self.context['view'].
    request.user, fields['purchaser'].queryset)
return fields


class Meta:
    pass
Dynamically limiting queryset of related field