NodeJS callback after multiple async-functions in for-loop - node.js

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

Answer 1

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" }
}}
])

Answer 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