[英]Issue in returning data retrieved from DB queries called in the loop
I making multiple mongoDB queries in loop. 我在循环中进行多个mongoDB查询。 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.
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。 I also tried to use q.moulde but same issue.
我也尝试过使用q.moulde但同样的问题。
Code: 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 is returned undefined. finalData返回undefined。
Let's start with the general rule for using promises: 让我们从使用promises的一般规则开始:
Every function that does something asynchronous must return a promise
每个执行异步操作的函数都必须返回一个promise
Which functions are these in your case? 在你的情况下,这些功能是什么? It's
getPrayerInCat
, the forEach
callback, and Prayer.find
. 它是
getPrayerInCat
, forEach
回调和Prayer.find
。
Hm, Prayer.find
doesn't return a promise, and it's a library function so we cannot modify it. 嗯,
Prayer.find
不返回一个promise,它是一个库函数,所以我们不能修改它。 Rule 2 comes into play: 规则2发挥作用:
Create an immediate wrapper for every function that doesn't
为每个没有的函数创建一个立即包装器
In our case that's easy with Q's node-interfacing helpers : 在我们的例子中, Q的节点接口助手很容易:
var find = Q.nbind(Prayer.find, Prayer);
Now we have only promises around, and do no more need any deferreds. 现在我们只有承诺,并且不再需要任何延期。 Third rule comes into play:
第三条规则发挥作用:
Everything that does something with an async result goes into a
.then
callback使用异步结果执行某些操作的所有内容都会转换为
.then
回调
…and returns the result. ...并返回结果。 Hell, that result can even be a promise if "something" was asynchronous!
地狱,如果“某事”是异步的,那么结果甚至可以成为一种承诺! With this, we can write the complete callback function:
有了这个,我们可以编写完整的回调函数:
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
});
}
Now, we have something a bit more complicated: a loop. 现在,我们有一些更复杂的东西:一个循环。 Repeatedly calling
getPrayerCount()
will get us multiple promises, whose asynchronous tasks run in parallel and resolve in unknown order. 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。 We want to wait for all of them - ie get a promise that resolves with all results when each of the tasks has finished. 我们希望等待所有这些 - 即获得一个在每个任务完成时解析所有结果的承诺。
For such complicated tasks, don't try to come up with your own solution: 对于这些复杂的任务,不要试图提出自己的解决方案:
Check the API of your library
检查您的库的API
And there we find Q.all
, which does exactly this. 在那里我们找到
Q.all
,它正是这样做的。 Writing getPrayerInCat
is a breeze now: 现在写
getPrayerInCat
是一件轻而易举的事:
function getPrayerInCat(data) {
var promises = data.map(getPrayerCount); // don't use forEach, we get something back
return Q.all(promises);
// ^^^^^^ Rule 1
}
If we needed to do anything with the array that Q.all
resolves to, just apply Rule 3. 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。
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