yii\db\BaseActiveRecord::delete PHP Method

    public function delete()
    {
        $result = false;
        if ($this->beforeDelete()) {
            // we do not check the return value of deleteAll() because it's possible
            // the record is already deleted in the database and thus the method will return 0
            $condition = $this->getOldPrimaryKey(true);
            $lock = $this->optimisticLock();
            if ($lock !== null) {
                $condition[$lock] = $this->{$lock};
            }
            $result = static::deleteAll($condition);
            if ($lock !== null && !$result) {
                throw new StaleObjectException('The object being deleted is outdated.');
            }
            $this->_oldAttributes = null;
            $this->afterDelete();
        }
        return $result;
    }

 public function afterDelete($event)
 {
     $this->_relation->delete();
 }