$baseQuerySql构建的子查询仍然是模型Sign对应的数据

public function mySign(Request $request)
{
    $baseQuerySql = $this->param($request, Sign::field('*'))
        ->where('user_id', $request->userId)
        ->order('create_date', 'DESC')
        ->limit(100)
        ->buildSql();

    /**@var $activity Collection **/
    $activity = Sign::table($baseQuerySql . ' a')->group('activity_id')->select();
   
}