NodeJS callback after multiple async-functions in for-loop

Question

I get a document from a mongodb which contains an array with comments for that document. In the comment is the _id of the user which wrote the comment.

I now need to get the username based on the _id of the user, but I'm running into several problems.

I have the following code which, obviously, doesn't work, but I hope it can give you an idea of what I'm trying to accomplish.

//MORE CODE... (No need to show this here, just a promise, some try catch and so on)
let article = await Article.findOne({_id:articleid})   
    for(var i = 0; i<=article.comment.length-1; i++){
        User.findOne({_id:article.comment[i].user}).then((user)=>{
            article.comment[i].username = user.username
        })
    }
return resolve(article)

I looked up several documentations but wasn't able to find a working solution. I tried using Promise.all, played around with a lot of async, await, tried to add a counter into the for-loop and resolve the promise after the loop finished but nothing worked so far.

This is what the article looks like in my db

{
    "_id" : ObjectId("5c18c1cbc47e5e29d42e4b0e"),
    "completed" : false,
    "completedAt" : null,
    "comment" : [ 
        {
            "_id" : ObjectId("5c18c95e328c8319ac07d817"),
            "comment" : "This is a comment",
            "rating" : [ ],
            "user" : ObjectId("5c18b76e73236d2168eda2b4")
        }, 
        {
            "_id" : ObjectId("5c18fb578de5741f20a4e2bd"),
            "comment" : "Another comment",
            "rating" : [ ],
            "user" : ObjectId("5c18b76e73236d2168eda2b4")
        }
    ]
}

I'm rather new to nodejs and mongodb aswell so I hope you can help a newbie like me.

Thank you for your Help

You can either await in the loop (each findOne) or await a Promise.all over a map instead of a `loop. That said, the actual correct way to do this in Mongo would be to bulk-update and not find and update the documents one-by-one — Benjamin Gruenbaum
– Benjamin Gruenbaum, Commented Dec 20, 2018 at 9:47
This is what mongoosejs.com/docs/populate.html is for, or you can attempt to use mongo's aggregation framework if you're feeling suicidal — nanobar
– nanobar, Commented Dec 20, 2018 at 10:05

Ashh · Accepted Answer · 2018-12-20 10:03:35Z

There are serveral approaches you can use here based on your convenience

Using async await

let article = await Article.findOne({ _id: articleid }).lean().exec()

await Promise.all(
  article.comment.map(async(obj) => {
    const user = await User.findOne({ _id: obj.user })
    obj.username = user.username
  })
)

console.log(article)

Using $lookup aggregation 3.6

Since mongodb has its own powerfull $lookup aggregation operator to join multiple collection and probably the better approach without any iteration

Article.aggregate([
  { "$match": { "_id": mongoose.Types.ObjectId(articleid) }},
  { "$unwind": "$comment" },
  { "$lookup": {
    "from": "users",
    "let": { "userId": "$comment.user" },
    "pipeline": [
      { "$match": { "$expr": { "$eq": ["$$userId", "$_id"] }}}
    ],
    "as": "comment.user"
  }},
  { "$unwind": "$comment.user" },
  { "$group": {
    "_id": "$_id",
    "comment": { "$push": "$comment" },
    "completed": { "$first": "$completed" },
    "completedAt": { "$first": "$completedAt" }
  }}
])

Using $lookup aggregation 3.4

Article.aggregate([
  { "$match": { "_id": mongoose.Types.ObjectId(articleid) }},
  { "$unwind": "$comment" },
  { "$lookup": {
    "from": "users",
    "localField": "comment.user",
    "foreignField": "_id",
    "as": "comment.user"
  }}
  { "$unwind": "$comment.user" },
  { "$group": {
    "_id": "$_id",
    "comment": { "$push": "$comment" },
    "completed": { "$first": "$completed" },
    "completedAt": { "$first": "$completedAt" }
  }}
])

Both those approaches worked like a charm. Thank you very much for you quick and efficient help :)

RamThakur · Accepted Answer · 2018-12-20 10:05:51Z

-2

You can try like the following way

const d = {
    "_id" : ObjectId("5c18c1cbc47e5e29d42e4b0e"),
    "completed" : false,
    "completedAt" : null,
    "comment" : [ 
        {
            "_id" : ObjectId("5c18c95e328c8319ac07d817"),
            "comment" : "This is a comment",
            "rating" : [ ],
            "user" : ObjectId("5c18b76e73236d2168eda2b4")
        }, 
        {
            "_id" : ObjectId("5c18fb578de5741f20a4e2bd"),
            "comment" : "Another comment",
            "rating" : [ ],
            "user" : ObjectId("5c18b76e73236d2168eda2b4")
        }
    ]
}

d.comment.forEach( async (obj, index) => {
    await new Promise((res) => {
            obj.counter = index;
            res();
    })
});

console.log(d);

For reference please take a look on following link Asycn/Await using forEach

edited Dec 20, 2018 at 10:05

answered Dec 20, 2018 at 9:58

RamThakur

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5 Comments

crellee Over a year ago

So in case of you have a list of 100 comments, it will take 100 seconds? You should avoid doing this.

RamThakur Over a year ago

``` setTimeout(() => { obj.counter = index; res(); }, 1000);`` the piece of code is for reference purpose not the actual one. I just gave an idea how can we do this. I didn't add the full-fledged code to copy paste. I think it's quite obvious to avoid timeout inside a loop :)

crellee Over a year ago

It makes sense. But it is still misleading.

Ashh Over a year ago

Why to use new Promise as async function itself returns promise.

RamThakur Over a year ago

yes async default returns promises, but before updating the above answer used with setTimout fn which by default doesn't return promises as @crellee feels that above example with setTimeout make it confusing

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