def _extract_simple_relation_keys(self, column_name: str) -> List:
        """
        Extracts simple relation keys values.

        :param column_name: names of the column(s) that holds the relation info
        :type column_name: str
        :return: List of extracted values of relation columns
        :rtype: List
        """
        list_of_ids = UniqueList()
        for row in self.rows:
            if row[column_name]:
                list_of_ids.append(row[column_name])
        return list_of_ids

        Extracts simple relation keys values.

        :param column_name: names of the column(s) that holds the relation info
        :type column_name: str
        :return: List of extracted values of relation columns
        :rtype: List
        
ormar/queryset/queries/prefetch_query.py