yii 2 Active Query中的数学运算

有没有一种方法可以将int列中的值减去1？我有一个按钮，当单击该按钮时，它将调用一个函数以将数据保存在表中，但是我想知道如何在a中减去一个数字使用yii 2的数据库我已经在下面尝试了以下代码，但是没有任何效果。有人可以帮我如何在yii 2中进行数学运算吗？

public static function AddSubject($subjectid, $clientid){
    $subject = ActiveCurriculum::findOne(['subjectid' => $subjectid]);

    $activesubject = new ActiveSubject();
    $activesubject->clientid = $clientid;
    $activesubject->subjectid = $subject->subjectid;
    $activesubject->subjectcode = $subject->subjectcode;
    $activesubject->days = $subject->days;
    $activesubject->time = $subject->time;
    $activesubject->section = $subject->section;
    $activesubject->room = $subject->room;
    $activesubject->units = $subject->units;
    $subject->units = $subject->units - 1; //this should subtract the number 
    //of slots by 1 but now it is not working.
    $activesubject->save();

    return true;

    //return static::findOne(['subjectid' => $subjectid]);
                //->where(['subjectid' => $subjectid]);

  }