使用事务代码报错：Call to a member function transaction() on a non-object [ 求助交流 ]

Booksear::findOne(['useflag' => $useflag]);没有返回记录，事务报错，我应该怎么写？

$model=Booksear::findOne(['userid' => Yii::$app->user->id,'useflag' => $useflag]);//用户书籍
if(!$model){
    $connection->transaction(function() {
        //抽取一个书籍
        $model=Booksear::findOne(['useflag' => $useflag]);
        if($model){
            
                //书籍存在，分配给用户
                $model->userid=Yii::$app->user->id;
                $model->save();
        }else{
            //书籍审核完毕,抽取待定书籍
            $useflag=3;
            $model=Booksear::findOne(['useflag' => $useflag]);//用户书籍
            if($model){	
                
                $connection->transaction(function() {
                   //书籍存在，分配给用户
                    if($model->userid!=Yii::$app->user->id){
                        $model->userid=Yii::$app->user->id;
                        $model->save();
                    }
                },Transaction::SERIALIZABLE);
                
            }else{
                //待定书籍审核完毕，返回列表页
                 return $this->redirect(['index']);
            }
        }
    },Transaction::SERIALIZABLE);
}
return $this->redirect(['inview', 'id' => $model->id]);