Why "as" in $lookup is replacing the complete set? - mongodb

Question

Let me first introduce you to the 2 collections I am using :
Collection 1 : users
> db.users.find().pretty()
{
"_id" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"name" : "Ashutosh Tiwari",
"age" : 21,
"email" : "ashutosh#gmail.com"
}
{
"_id" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"name" : "Maximilian",
"age" : 32,
"email" : "max#yahoo.com"
}
Collection 2 : posts
> db.posts.find().pretty()
{
"_id" : ObjectId("5ee51b7ed9f661cad505fcc6"),
"title" : "First One",
"text" : "Hey this is the first Author",
"author" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"comments" : [
{
"user" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"comment" : "This is my comment"
}
]
}
{
"_id" : ObjectId("5ee5353cd9f661cad505fcc8"),
"title" : "First One",
"author" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"comments" : [
{
"user" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"comment" : "This is my comment"
}
]
}
I want to have the user inside comments array in 2nd Collection(posts) to be replaced by the user who has written that comment.
I have tried the query below but it is replacing the comments section !
> db.posts.aggregate([
{ $lookup:
{from: "users",
localField:"comments.user",
foreignField:"_id",
as:"comments.user"
}
} ]).pretty()
{
"_id" : ObjectId("5ee51b7ed9f661cad505fcc6"),
"title" : "First One",
"text" : "Hey this is the first Author",
"author" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"comments" : {
"user" : [
{
"_id" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"name" : "Ashutosh Tiwari",
"age" : 21,
"email" : "ashutosh#gmail.com"
}
]
}
}
{
"_id" : ObjectId("5ee5353cd9f661cad505fcc8"),
"title" : "First One",
"author" : ObjectId("5ee4e727d04e4b4ac1ef115c"),
"comments" : {
"user" : [
{
"_id" : ObjectId("5ee4e727d04e4b4ac1ef115b"),
"name" : "Ashutosh Tiwari",
"age" : 21,
"email" : "ashutosh#gmail.com"
}
]
}
}
So, here, whole comments section is now replaced whereas I wanted to have the details in comments.user section so I could see the comment and the user who has posted that comment.

Answer 1

you need to unwind the comments array first
your query may look something like this
db.posts.aggregate([
{
$unwind: "$comments" // unwind the comments array to get a stream of documents, each document has only one comment
},
{
$lookup: {
from: "users",
localField: "comments.user",
foreignField: "_id",
as: "comments.user"
}
},
{
$unwind: "$comments.user" // we know there is only one user inside a single comment, so we can unwind this user array to be an object too (as the lookup returns an array)
},
{
$group: { // then do a group by the document _id to get unique documents with comments array instead of the same document duplicated with different comments
_id: "$_id",
author: {
$first: "$author"
},
text: {
$first: "$text"
},
title: {
$first: "$title"
},
comments: {
$push: "$comments"
}
}
}
])
you can test it here
hope it helps

Answer 2

You can handle it in the projection.
db.posts.aggregate([
{ $lookup:
{from: "users",
localField:"comments.user",
foreignField:"_id",
as:"cu"
}
},
{$unwind:{path:"$cu"}},
{
$project:{
"title":1,
"text":1,
"author":1,
"comments":{
user: "$cu",
comment: { $arrayElemAt: [ "$comments.comment", 0 ] },
}
}
}
])