返回从循环中调用的数据库查询中检索到的数据的问题

2022-01-30 00:00:00 mongodb node.js javascript promise q

我在循环中进行了多个 mongoDB 查询.并希望将所有结果作为一个数据数组发送.但是当我简单地使用 return 发送数据时，它只是返回 undefined 并且不等待所有 DB 请求的结果.我也尝试使用 q.moulde 但同样的问题.

I making multiple mongoDB queries in loop. and want to send the all results as one data array.But when I simple use the return for send the data it simply return undefined and do not wait for results of all DB request. I also tried to use q.moulde but same issue.

代码:

var getPrayerInCat = function(data){
    var result ;
    var finalData = [];
    if(data.length >0){
             data.forEach(function(data2){
                 var id= data2.id;
                 Prayer.find({prayerCat:id},function(err,prayer){
                     var deferred = Q.defer()
                     if (err) { // ...
                         console.log('An error has occurred');
                         // res.send(err);
                         result= finalData = err
                     } else {
                         if(!prayer){
                             // console.log(data2.id+'--0');
                             data2.prayersCount = 0;
                             result = deferred.resolve(finalData.push(data2))
                         } else {
                             // console.log(data2.id+'--'+prayer.length);
                             data2.prayersCount = prayer.length;
                             // console.log(prayer)
                             result =  deferred.resolve(finalData.push(data2))
                         } // else for data forward
                     }
                     deferred.promise;
                 })
                // deferred.resolve(finalData);

             })
             /*if(finalData.length > 0) { return finalData;}*/
        }
}

finalData 返回未定义.

finalData is returned undefined.

推荐答案

让我们从使用 Promise 的一般规则开始:

Let's start with the general rule for using promises:

每个执行异步操作的函数都必须返回一个承诺

在您的情况下，这些功能是什么?它是 getPrayerInCat、forEach 回调和 Prayer.find.

Which functions are these in your case? It's getPrayerInCat, the forEach callback, and Prayer.find.

嗯，Prayer.find 不返回一个承诺，它是一个库函数，所以我们不能修改它.规则 2 发挥作用:

Hm, Prayer.find doesn't return a promise, and it's a library function so we cannot modify it. Rule 2 comes into play:

为不支持的每个函数创建一个立即包装器

在我们的例子中，使用 Q 的节点接口助手:

In our case that's easy with Q's node-interfacing helpers:

var find = Q.nbind(Prayer.find, Prayer);

现在我们只有 Promise，不再需要任何 deferred.第三条规则发挥作用:

Now we have only promises around, and do no more need any deferreds. Third rule comes into play:

所有对异步结果执行的操作都会进入 .then 回调

Everything that does something with an async result goes into a .then callback

...并返回结果.地狱，如果某事"是异步的，那么这个结果甚至可以是一个承诺！这样，我们就可以编写完整的回调函数了:

…and returns the result. Hell, that result can even be a promise if "something" was asynchronous! With this, we can write the complete callback function:

function getPrayerCount(data2) {
    var id = data2.id;
    return find({prayerCat:id})
//  ^^^^^^ Rule 1
    .then(function(prayer) {
//  ^^^^^ Rule 3
        if (!prayer)
            data2.prayersCount = 0;
        else
            data2.prayersCount = prayer.length;
        return data2;
//      ^^^^^^ Rule 3b
    });
}

现在，我们有一些更复杂的东西:一个循环.重复调用 getPrayerCount() 会得到多个 Promise，它们的异步任务并行运行并以未知的顺序解析.我们希望等待所有这些 - 即，当每个任务完成时，得到一个解决所有结果的承诺.

Now, we have something a bit more complicated: a loop. Repeatedly calling getPrayerCount() will get us multiple promises, whose asynchronous tasks run in parallel and resolve in unknown order. We want to wait for all of them - i.e. get a promise that resolves with all results when each of the tasks has finished.

对于如此复杂的任务，不要试图想出自己的解决方案:

For such complicated tasks, don't try to come up with your own solution:

检查库的 API

我们在那里找到 Q.all，正是这样做的.现在编写 getPrayerInCat 变得轻而易举:

And there we find Q.all, which does exactly this. Writing getPrayerInCat is a breeze now:

function getPrayerInCat(data) {
    var promises = data.map(getPrayerCount); // don't use forEach, we get something back
    return Q.all(promises);
//  ^^^^^^ Rule 1
}

如果我们需要对 Q.all 解析为的数组做任何事情，只需应用规则 3.

If we needed to do anything with the array that Q.all resolves to, just apply Rule 3.

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