Return the value to insert in a relationship cell
        Handle the case of complex related datas we want to handle

def _get_relationship_cell_val(self, obj, column):
        """
        Return the value to insert in a relationship cell
        Handle the case of complex related datas we want to handle
        """
        val = SqlaExporter._get_relationship_cell_val(self, obj, column)
        if val == "":
            related_key = column.get('related_key', None)

            if column['__col__'].uselist and related_key is None and \
                    self.is_root:

                # on récupère les objets liés
                key = column['key']
                related_objects = getattr(obj, key, None)
                if not related_objects:
                    return ""
                else:
                    exporter = self._get_related_exporter(
                        related_objects[0],
                        column,
                    )
                    for rel_obj in related_objects:
                        exporter.add_row(rel_obj)

        return val