异步 Mongo 数据库查询

Question

In my code, I am sending a query to my Mongo database. The method findUser() shall return the response of this query. The query works fine, tested with console.log(users) . The problem is the function returns null , it doesn't wait till the query got a response to return the var foundUser . How could I use await/async in this case in order to wait for the query response before returning anything?

function findUser(username) { 
    foundUser = null
    const client = new MongoClient(uri, { useNewUrlParser: true, useUnifiedTopology : true});
    client.connect(err => {
        const collection = client.db("YourCV").collection("Accounts");
        result = collection.findOne({username : username }, function(err, user) {
            console.log(user)
            if(user){
                foundUser = user
            }
        });
    });  
    return foundUser
};

console.log(user) outputs:

{
  _id: 601695084b28102500ae0015,
  username: 'jetlime',
  password: '$2b$10$EN5k/YKOMBgibqy62s0hGOX9MffHZtXkfsw0Du0j8QVS7mGab5FLi'
}

Many thanks

Answer 1

Update the code to the following:

async function findUser(username) {
    const client = await MongoClient.connect(url, { useNewUrlParser: true })
        .catch(err => { console.log(err); });

    if (!client) {
        return;
    }
    const collection = client.db("YourCV").collection("Accounts");
    const user = await collection.findOne({username : username });
    client.close(); // -> To be under finally clause.
    return user;
};

And call the function with await findUser(username);

Caution: The above of connecting to DB is not recommended. You are establishing a DB connection for every function call. This quickly leads to running out of connections on the DB side when you have a large number of requests .

Move the DB connection establishing part to a commonplace and re-use the connection.

Answer 2

See whatever you do, any operation you perform with your database, it returns promise, so write async keyword before the name of your function in line 1 and then await keyword before your query in line 6, it will then behave like synchronous call and every line will execute accordingly

Answer 3

As you said, you need to use async/await operators for asynchron methods.

You have two choices:

1)Implement callback method on findUser

  async function findUser(username, onUsersReady) {
    const client = new MongoClient(uri, {
      useNewUrlParser: true,
      useUnifiedTopology: true
    }).connect();

    const collection = await client.db("YourCV").collection("Accounts");
    await collection.findOne({
      username: username
    }, function(err, user) {
      console.log(user)
      if (user) {
        foundUser = user
        onUsersReady(user);
      }
    });
  };

2)Use function to return results directly

async function findUser(username) {
  const client = await new MongoClient(uri, {
    useNewUrlParser: true,
    useUnifiedTopology: true
  }).connect();

  const collection = await client.db("YourCV").collection("Accounts");
  const foundUser = await collection.findOne({
    username
  });
  return foundUser;
}