返回从循环中调用的数据库查询中检索的数据的问题 [英] Issue in returning data retrieved from DB queries called in the loop
问题描述
我在循环中进行多个mongoDB查询。并且想要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来发送数据时，它只返回undefined而不等待所有数据库请求的结果。我也试过使用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;}*/
}
}
未定义返回finalData。
finalData is returned undefined.
推荐答案
让我们从使用promises的一般规则开始：
Let's start with the general rule for using promises:
执行异步操作的每个函数都必须返回一个承诺
您的案例中有哪些功能？它是 getPrayerInCat ， forEach 回调， Prayer.find 。
Which functions are these in your case? It's getPrayerInCat, the forEach callback, and Prayer.find.
Hm， Prayer.find 没有返回一个承诺，它是一个库函数，所以我们无法修改它。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:
Create an immediate wrapper for every function that doesn't
在我们的情况下，使用< a href =https://github.com/kriskowal/q/wiki/API-Reference#qnbindnodemethod-thisarg-args\"rel =nofollow noreferrer> 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);
现在我们只有承诺，不再需要任何延期。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:
使用异步结果执行某些操作的所有内容都会进入
。然后回调
Everything that does something with an async result goes into a
.thencallback
...并返回结果。地狱，如果某事是异步的，那么结果甚至可以成为一种承诺！有了这个，我们可以编写完整的回调函数：
…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。反复调用 getPrayerCount（）将获得多个promise，其异步任务并行运行并以未知顺序解析。我们希望等待所有这些 - 即获得一个承诺，当每个任务完成时，它会解决所有结果。
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.
对于这些复杂的任务，不要尝试提出自己的解决方案：
For such complicated tasks, don't try to come up with your own solution:
检查您图书馆的API
我们发现 Q.all ，就是这样做的。写 getPrayerInCat 现在是轻而易举的事了：
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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