Nested MongoDB query in NodeJS

Question

I want to select from two collections in MongoDB with NodeJS. I select from the chat_messages collection, there is a userId property, and I would like to extend the resulted object with the user name with the help of ES6 Promise. I tried this:

db.collection("chat_messages")
    .find({"room" : roomName})
    .sort({"created" : 1})
    .toArray()
    .then(function(messages){
        console.log(messages);
        return Promise.all(messages.map(function(message){
            return db.collection("chat_users")
                .find({"id" : message.userId})
                .limit(1)
                .toArray()
                .then(function(users){
                    message.userName = users[0].name;
                });
        }));
    })
    .then(function(messages){
        console.log(messages);
    })
    .catch(function(error){
        // ...
    });

The first console.log prints this:

[
    {
        _id: 573b6f2af9172fd81252c520,
        userId: 2,
        ...
    },
    {
        _id: 57388bd913371cfc13323bbb,
        userId: 1,
        ...
    }
]

But the second looks like this:

[ undefined, undefined ]

What am I mess up?

Answer 1

Promise.all returns data passed into a resolve function of a promise. this should work

db.collection("chat_messages")
    .find({"room" : roomName})
    .sort({"created" : 1})
    .toArray()
    .then(function(messages){
        let promises = [];

        messages.forEach(message => {
            promises.push(new Promise(resolve => {
                db.collection("chat_users")
                    .find({"id" : message.userId})
                    .limit(1)
                    .toArray()
                    .then(function(users){
                        message.userName = users[0].name;
                        resolve(message);
                    });
            }));
        });
        return Promise.all(promises);
    })
    .then(function(messages){
        console.log(messages);
    })
    .catch(function(error){
        // ...
});