Return tha parent_lookup_field for the NestedHyperlinkedModelSerializer.
        If `parent_lookup_field` of `parent_model` is not defined lookup ForeignKey field of model.
        :return: parent_lookup_field

def get_parent_lookup_field(self):
        """
        Return tha parent_lookup_field for the NestedHyperlinkedModelSerializer.
        If `parent_lookup_field` of `parent_model` is not defined lookup ForeignKey field of model.
        :return: parent_lookup_field
        """
        if hasattr(self.Meta, "parent_lookup_field"):
            return getattr(self.Meta, "parent_lookup_field")
        elif hasattr(self.Meta, "parent_model"):
            model = getattr(self.Meta, "parent_model")
        else:
            parent_models = filter(
                lambda field: isinstance(field, db_models.ForeignKey),
                self.Meta.model._meta.fields
            )
            assert len(parent_models) > 0, self.default_error_messages['no_parent']
            assert len(parent_models) == 1, self.default_error_messages['multiple_parents']
            model = parent_models[0].related_model

        return '%(model_name)s__pk' % {
            'app_label': model._meta.app_label,
            'model_name': model._meta.object_name.lower()
        }