从 .then 中的循环中获取值 nodejs 在 allemail 数组中

Question

// importing required builtin modules
var mongoose = require('mongoose');
mongoose.connect('mongodb://localhost/mydb');
// schema for email
var emailSchema = mongoose.Schema({
    _id: mongoose.Schema.Types.ObjectId,
    html: String,
    text: String,
    headers: {},
    subject: String,
    references: [String],
    messageId: String,
    inReplyTo: [String],
    priority: String,
    from: [],
    replyto: [String],
    to: [],
    date: Date,
    receivedDate: Date,
    attachments: [],
    read: { type: Boolean, default: 0 },
    createdAt: { type: Date, default: Date.now },
    updatedAt: { type: Date, default: Date.now },
    active: { type: Boolean, default: 1 },
    labels: [String]
});
// schema for thread
var threadSchema = mongoose.Schema({
    threadedEmails: [{ type: String, ref: 'Email' }],
    createdAt: { type: Date, default: Date.now },
    updatedAt: { type: Date, default: Date.now }
});
// defining models
var Email = mongoose.model('Email', emailSchema);
var Thread = mongoose.model('Thread', threadSchema);
module.exports = Email;
module.exports = Thread;
// function to return an array which contains yet anohter array of emails each representing a thread
function doCalls() {
    threads = [];
    // a promise that always resolves
    return new Promise(function (resolve, reject) {
    resolve(1);
    })
    // this returns the threads as expected
    .then(function (result) {
    return Promise.resolve(
        Thread.find({}, { threadedEmails: 1, _id: 0 }).then(
            (_threads) => { return _threads }, //resolve
            (err) => { reject(err); } //reject
        )
    )
    })
    // this does not returns the emails array as i expect
    .then(function (threads) {
    allEmails = [];
    threads.forEach(thread => {
        // Start off with a promise that always resolves
        var sequence = Promise.resolve();
        sequence = sequence.then(function (result) {
            console.log('then of first foreach');
            //query to make a database call to get all the emails whoes messageId's matchs
            query = Email.find({ messageId: { "$in": thread.threadedEmails } });
            query.exec((err, result) => {
                if (err) throw err;
                allEmails.push(result); //but this does not works because the code execution moves ahead
                console.log(result);    //this console log returns the value
            });
        })
    })
    //----------------- this is the problematic code here this array returns empty ----------------//
    console.log(allEmails);
    })
}
doCalls()
    .then(function (allEmails) {
    // console.log(allEmails);
    });

我已经在我认为代码中需要的地方写了注释，即使让我解释一下我正在尝试的上下文

• 我正在从一个名为线程的集合中获取所有成功运行的线程
• 之后，我尝试使用数据库查询从名为 email 的集合中获取所有电子邮件，我尝试将其输出存储在名为 allEmails 的数组中
• 如果我在数据库调用后立即在 .then(( 中控制台.log(( 它会给我输出，
• 我的问题是我如何把它弄出来？
• 我想要一个包含另一个数组的数组，每个数组都有一个来自电子邮件集合的电子邮件集合(每个代表一个线程(

希望我说清楚，如果还有其他我需要提供的东西，请成为我的向导并告诉我。

Answer 1

forEach 块在其内部的任何异步操作返回之前完成。因此，当您控制台时，您的allEmails数组仍将为空.log。

你需要做的是构建一个承诺数组，这些承诺通过"结果"解析，然后将这个数组输入到Promise.all(arrayOfPromises(中，当你的所有承诺都解决时，它将解析。

Promise.all非常友好地为您解决一系列结果。您的最后一个看起来像这样：

.then(function (threads) {
  const promiseArray = [];
  threads.forEach(thread => promiseArray.push(
      Promise.resolve(Email.find({ messageId: { "$in": thread.threadedEmails } }))
    )
  );
  return Promise.all(promiseArray);
}

话虽如此，对于您要实现的目标，Model.aggregate((将是一个更优雅的解决方案。

希望这有帮助