使用es7 async/await检查mongodb中是否存在文档 [英] Check if document exists in mongodb using es7 async/await

问题描述

我试图检查提供了email的用户是否存在于集合users中，但是我的函数始终为每个调用返回未定义的内容.我使用es6和async/await来摆脱很多回调.这是我的功能(在类的内部):

I'm trying to check if the user with the email provided exists in the collection users, but my function keeps returning undefined for every call. I use es6 and async/await in order to get rid of lots of callbacks. Here's my function (it is inside of a class):

async userExistsInDB(email) {
    let userExists;
    await MongoClient.connect('mongodb://127.0.0.1:27017/notificator', (err, db) => {
        if (err) throw err;

        let collection = db.collection('users');

        userExists = collection.find({email: email}).count() > 0;
        console.log(userExists);

        db.close();
    });
    console.log(userExists);
    return userExists;
}

因此，在.connect调用内的第一个console.log总是返回false，因为.find的返回值不是数组，而是一个看起来像这样的巨大对象:

So, the first console.log inside the .connect call always returns false because the returned value of the .find is not an array, it is some huge object which looks like this:

{ connection: null,
  server: null,
  disconnectHandler: 
   { s: { storedOps: [], storeOptions: [Object], topology: [Object] },
     length: [Getter] },
  bson: {},
  ns: 'notificator.users',
  cmd: 
   { find: 'notificator.users',
     limit: 0,
     skip: 0,
     query: { email: '[e-mail]' },
     slaveOk: true,
     readPreference: { preference: 'primary', tags: undefined, options: undefined } },
  options: 
........
........

最后一个console.log总是不确定的(尽管我认为不应该这样，因为await等待异步调用的结束，对吧?)

And the last console.log is always undefined (although I think it shouldn't be like that, because await waits for the end of the asynchronous call, right?)

我只需要函数即可返回布尔值，而不是Promise之类的东西.

I just need my function to return a boolean value, not a Promise or something.

有人可以帮我吗?

console.log(collection.findOne({email: email}));会返回以下内容:

 { 'Symbol(record)_3.ugi5lye6fvq5b3xr': 
   { p: [Circular],
     c: [],
     a: undefined,
     s: 0,
     d: false,
     v: undefined,
     h: false,
     n: false } }

---

更新2

似乎是由于我对es7 async/await的了解不足而引起的问题.

---

UPDATE 2

Seems like it was the issue with my poor knowledge of es7 async/await.

现在.connect中的代码将返回所需的值.

Now the code inside the .connect returns the needed value.

async userExistsInDB(email) {
    let userExists;
    await* MongoClient.connect('mongodb://127.0.0.1:27017/notificator', async(err, db) => {
        if (err) throw err;

        let collection = db.collection('users');
        userExists = await collection.find({email: email}).limit(1).count() > 0;

        db.close();
    });
    console.log(userExists); // <--- this is not called at all
    return userExists;
}

但是，现在完全不执行console.log或.connect调用之后的任何操作.

However, now the console.log or anything after the .connect call is not performed at all.

现在，每次我在某个地方调用userExistsInDB()函数并console.log其结果时，我都会得到:

Now, every time I call the userExistsInDB() function somewhere and console.log its result, I get this:

 { 'Symbol(record)_3.78lmjnx8e3766r': 
   { p: [Circular],
     c: [],
     a: undefined,
     s: 0,
     d: false,
     v: undefined,
     h: false,
     n: false } }

有什么想法为什么会这样?

Any ideas why it is like that?

推荐答案

好的，这就是我的工作方式:

Ok, here's how I got it working:

async function userExistsInDB(email, password) {
    let db = await MongoClient.connect('mongodb://127.0.0.1:27017/notificator');
    try {
        let collection = db.collection('users');
        let userCount = (await collection.find(
            {
                email: email,
                password: password
            }).limit(1).count());
        return userCount > 0;
    } finally {
        db.close();
    }
}

并且由于函数声明保证中的async关键字返回的值将是Promise，因此从该函数中获取实际返回结果的唯一方法是:

And because the async keyword in function declaration guarantees that the returned value will be a Promise, the only way to get the real returned result out of this function is:

let result = await this.userExistsInDB(email, password);在另一个声明为async的函数中.

let result = await this.userExistsInDB(email, password); inside of another function declared async.

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