node.js - 异步查询node.js中的数据库

Question

如果函数也对数据库进行异步调用，我正在尝试找出异步调用函数的正确方法。

在我的代码中，我调用了一个名为“check_foods()”的函数，如果尚未调用该函数，它将最多查询数据库两次。数据库查询将填充蔬菜和水果这两个变量(假设它们尚未填充)。一旦函数返回，我想使用这些变量。

但是，我假设我的代码现在的方式是尝试开始处理两条 console 行，即使它还没有从数据库中提取数据。我的这个假设是否正确？如果是这样，我应该怎么做才能正确实现？

我的代码的简化版本如下所示:

var vegetables = {};
var fruit = {};


async function starter (){
    sql = "select growth_items from garden where room_id = ?";
    db_connection.query(sql, [socket.room], function (err, result){
        if (err){
            console.log("Garden Error: " + err);
            return;
        }

        console.log(result);

        check_foods();

        //Should not get to this point until variables have been populated
        //However, due to the async nature of the code this does not seem to be implemented correctly. 
        //process vegetables[socket.room] and fruit[socket.room]
        console.log(vegetables[socket.room]);
        console.log(fruit[socket.room]);
    });    
}

async function check_foods(){
       //Check to see if vegetables[socket.room] already exists, otherwise load it from the database
       if (typeof vegetables[socket.room] !== "undefined"){
            sql = "select name, qty from vegetables where room_id = ?";
            db_connection.query(sql, [socket.room], function (err, result){
                if (err){
                    console.log("An vegetable error has occurred: " + err);
                    return;
                }

                vegetables[socket.room] = result;
            });
        };

        //Check to see if fruit already exists before looking up again

        if (typeof fruit[socket.room] !== "undefined"){
            sql = "select name, qty from fruit where room_id = ?";
            db_connection.query(sql, [socket.room], function (err, result){
                if (err){
                    console.log("An fruit error has occurred: " + err);
                    return;
                }

                fruit[socket.room] = result;
            });
        };

}

Answer 1

将所有内容分解成小单元对此类项目非常有帮助。这样，异步部分就可以以更易于推理的方式包含或 promise 。看起来你想使用 async/await 这对这个很好，但这意味着你需要生成 promise 。上次我检查标准 MySQL 库不会这样做，但包装它们很容易。

所以我会把它分解成这样:

function check_growth_items(room){
   //simple function to get growth items
    return new Promise((resolve, reject) => {
        sql = "select growth_items from garden where room_id = ?";
        db_connection.query(sql, [room], function (err, result){
            if (err){
                console.log("Garden Error: " + err);
                reject(err);
            }
            resolve(result)
        })
    })
}

function get_food(type, room_id){
    // expects a type [vegetable | fruit] and room
    return new Promise((resolve, reject) => {
        sql = "select name, qty from ? where room_id = ?";
        db_connection.query(sql, [type, room_id], function (err, result){
            if (err){
                console.log("An vegetable error has occurred: " + err);
                reject(err);
            }
            resolve(result);
        });
    })
}

现在您所有的异步内容都很简单。在异步函数中，您可以调用如下内容:

let items = await check_growth_items(socket.room)

或获取食物:

fruit[socket.room] = await get_food('fruit',socket.room )

或

get_food('fruit',socket.room )
.then(result => fruit[socket.room] = result)
.catch(err => console.log(err))

诚然，我不是 100% 确定你的最终代码应该做什么，但你的主要功能应该看起来像:

async function starter (){
    // it's not clear in your code how you're using this
    // of it the other async calls depend on the return value
    var growth_items = await check_growth_items(socket.room)
    if (!vegetables[socket.room]) {
        vegetables[socket.room] = await get_food('vegetables',socket.room )
    }
    if (!fruit[socket.room]) {
        fruit[socket.room] = await get_food('fruit',socket.room )
    }

    console.log(vegetables[socket.room]);
    console.log(fruit[socket.room]);
} 

这可能不是剪切和粘贴的解决方案，但希望能给您一些关于更好地组织片段的想法。