在变量中持有承诺的未来价值

皮特

我有一个数据库,正在要求最近的邮件列表。每个消息都是一个对象,并作为这些消息对象的Array存储在chatListNew中。

每个消息对象都有一个属性“来自”,这是发布该消息的用户的ID。我要做的是遍历此数组,并将“发件人”用户的实际配置文件信息附加到对象本身中。这样,当前端接收到信息时,它就可以访问相应邮件的fromProfile属性中的一个特定邮件的发件人的个人资料。

我曾想过要遍历每个人并做出一个Promise,但是如果只有很少的用户发布数百条消息,那么这对于每个人来说都是非常昂贵的。为每个用户只运行一次猫鼬查询会更有意义因此,我发明了一种缓存系统。

但是,我对如何在数组元素中存储将来值的承诺感到困惑。我以为将“ fromProfile”设置为先前调用的promise会神奇地保留此promise,直到该值被解析为止。因此,我使用Promise.all来确保所有的诺言都已完成,然后由结果返回,但是我存储在数组中的诺言不是我期望的值。

这是我的代码:

//chatListNew = an array of objects, each object is a message that has a "from" property indicating the person-who-sent-the-message's user ID

let cacheProfilesPromises = []; // this will my basic array of the promises called in the upcoming foreach loop, made for Promise.all
let cacheProfilesKey = {}; // this will be a Key => Value pair, where the key is the message's "From" Id, and the value is the promise retrieving that profile
let cacheProfileIDs = []; // this another Key => Value pair, which basically stores to see if a certain "From" Id has already been called, so that we can not call another expensive mongoose query


chatListNew.forEach((message, index) => {
    if(!cacheProfileIDs[message.from]) { // test to see if this user has already been iterated, if not
        let thisSearch = User.findOne({_id : message.from}).select('name nickname phone avatar').exec().then(results => {return results}).catch(err => { console.log(err); return '???' ; }); // Profile retrieving promise
        cacheProfilesKey[message.from] = thisSearch;
        cacheProfilesPromises.push(thisSearch); // creating the Array of promises
        cacheProfileIDs[message.from] = true;
    }

    chatListNew[index]["fromProfile"] = cacheProfilesKey[message.from]; // Attaching this promise (hoping it will become a value once promise is resolved) to the new property "fromProfile"
});

Promise.all(cacheProfilesPromises).then(_=>{ // Are all promises done?
    console.log('Chat List New: ', chatListNew);
    res.send(chatListNew);
});

这是我的控制台输出:

Chat List New:  [ { _id: '5b76337ceccfa2bdb7ff35b5',
updatedAt: '2018-08-18T19:50:53.105Z',
createdAt: '2018-08-18T19:50:53.105Z',
from: '5b74c1691d21ce5d9a7ba755',
conversation: '5b761cf1eccfa2bdb7ff2b8a',
type: 'msg',
content: 'Hey everyone!',
fromProfile:
 Promise { emitter: [EventEmitter], emitted: [Object], ended: true } },
{ _id: '5b78712deccfa2bdb7009d1d',
updatedAt: '2018-08-18T19:41:29.763Z',
createdAt: '2018-08-18T19:41:29.763Z',
from: '5b74c1691d21ce5d9a7ba755',
conversation: '5b761cf1eccfa2bdb7ff2b8a',
type: 'msg',
content: 'Yo!',
fromProfile:
 Promise { emitter: [EventEmitter], emitted: [Object], ended: true } } ]

而我希望有这样的东西:

Chat List New:  [ { _id: '5b76337ceccfa2bdb7ff35b5',
updatedAt: '2018-08-18T19:50:53.105Z',
createdAt: '2018-08-18T19:50:53.105Z',
from: '5b74c1691d21ce5d9a7ba755',
conversation: '5b761cf1eccfa2bdb7ff2b8a',
type: 'msg',
content: 'Hey everyone!',
fromProfile:
 Promise {name: xxx, nickname: abc... etc} },
{ _id: '5b78712deccfa2bdb7009d1d',
updatedAt: '2018-08-18T19:41:29.763Z',
createdAt: '2018-08-18T19:41:29.763Z',
from: '5b74c1691d21ce5d9a7ba755',
conversation: '5b761cf1eccfa2bdb7ff2b8a',
type: 'msg',
content: 'Yo!',
fromProfile:
 {name: xxx, nickname: abc... etc} } ]

感谢你们!对实现这一目标的其他方式持开放态度:)皮特

一定的表现

当将aPromise分配给变量时,该变量将始终为a Promise,除非重新分配该变量。你需要得到的结果你的Promises从你的Promise.all电话。

.then仅仅返回其参数也没有意义,就像您一样.then(results => {return results})-您可以完全忽略它,它什么也没做。

构造Promises数组,并构造from属性数组,以使每个Promise都from对应于另一个数组中相同索引处的项目。这样,Promise.all完成后,您可以将解析值数组转换为索引为的对象from,之后可以遍历chatListNew并将解析分配给fromProfile每个消息属性:

const cacheProfilesPromises = [];
const messagesFrom = [];

chatListNew.forEach((message, index) => {
  const { from } = message;
  if(messagesFrom.includes(from)) return;
  messagesFrom.push(from);
  const thisSearch = User.findOne({_id : from})
    .select('name nickname phone avatar')
    .exec()
    .catch(err => { console.log(err); return '???' ; });
  cacheProfilesPromises.push(thisSearch);
});

Promise.all(cacheProfilesPromises)
  .then((newInfoArr) => {
    // Transform the array of Promises into an object indexed by `from`:
    const newInfoByFrom = newInfoArr.reduce((a, newInfo, i) => {
      a[messagesFrom[i]] = newInfo;
      return a;
    }, {});

    // Iterate over `chatListNew` and assign the *resolved* values:
    chatListNew.forEach((message) => {
      message.fromProfile = newInfoByFrom[message.from];
    });
  });

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