Laravel 5.7 joinSub 子查询，如果有表前缀的话，别名怎么定义？

表前缀 lv

$subQuery = $this::query()
    ->from('t1 as a')
    ->getQuery();
$query = $this::query()
    ->from('t2 as b')
    ->joinSub($subQuery, 'sub', 'sub.id', '=', 'b.id');

生成的SQL语句

select * from `lv_t2` as `lv_b` inner join (select * from `lv_t1` as `lv_a`) as `sub` on `lv_sub`.`id` = `lv_b`.`id`

问题

生成的SQL语句的别名少了表前缀，难不成需要在代码里直接获取表前缀进行拼接吗？