Place an object to root level in mongoDb - node.js

Question

I have a aggregate query that fetches result from 3 collections.
I am using mongoDb 3.4
One Sample doc from result.
{
"_id" : ObjectId("5ba1717ee4b00ce08ca47cfa"),
"name" : "captain jack",
"email" : "jack#gmail.com",
"mobile" : "9000000023",
"status" : "verified",
"courses" : [
{
"_id" : "13",
"name" : "Course (03)"
},{
"_id" : "12",
"name" : "Course (03)"
}
],
"examCompleted" : false,
"login" : "5ba1717ee4b00fe08ca47cfa",
"partnerMetaInfo" : {
"_id" : ObjectId("5ba1717ee4b00fe08ca47cfa"),
"costCode" : "5761",
"hub" : "CALCUTTA",
"location" : "Kolkata"
}
}
I am trying to bring partnerMetaInfo at root level.
I am also unable to filter courses._id using $match on _id == 13
This is my aggregate query :
db.getCollection("mainCollection").aggregate([
{
//Join two collection
$lookup:{
from: "Details",
localField: "username",
foreignField: "login",
as: "partnerData"
}
},{
//Limit fields
$project:{
"email":1,
"name":1,
"mobile":1,
"status" : 1,
"courses":"$partnerData.courses",
"examScore" : "$partnerData.examScore",
"examCompleted" : "$partnerData.examCompleted",
"login":"$partnerData.login"
}
},
{
//Join third collection
$lookup:{
from: "PartnerMetaInfo",
localField: "login",
foreignField: "partnerId",
as: "partnerMetaInfo"
}
},
//Remove from partnerData array and place at root level.
{
$unwind:
{
path: '$courses',
preserveNullAndEmptyArrays: true
}
},{
$unwind:
{
path: '$examScore',
preserveNullAndEmptyArrays: true
}
},{
$unwind:
{
path: '$examCompleted',
preserveNullAndEmptyArrays: true
}
},{
$unwind:
{
path: '$login',
preserveNullAndEmptyArrays: true
}
},//Bring $partnerMetaInfo array to root level.
{
$unwind:
{
path: '$partnerMetaInfo',
preserveNullAndEmptyArrays: true
}
},{
$limit:10
}
];
partnerMetaInfo after $unwind ends up as object. I want to flatten it and bring it at root level.
Can any body help me with this?

Answer 1

If all you want to get as a result is the content of your partnerMetaInfo field then you can simply add a $replaceRoot stage at the end of your pipeline like this:
{
$replaceRoot: { "newRoot": { $ifNull: [ "$partnerMetaInfo", {} ] } }
}
Otherwise, in case you want to simply move the fields inside the partnerMetaInfo field to the root then you would use $addFields:
{
$addFields: {
"partnerMetaInfoId" : "$partnerMetaInfo._id",
"costCode" : "$partnerMetaInfo.costCode",
"hub" : "$partnerMetaInfo.hub",
"location" : "$partnerMetaInfo.location"
}
}
If you have a dynamic number of fields or do not want to hardcode field names then you can use the following logic:
{
$replaceRoot: { // merge fields of and move them all the way up
"newRoot": { $mergeObjects: [ "$$ROOT", "$partnerMetaInfo" ] }
}
}, {
$project: { // remove the "partnerMetaInfo" field
"partnerMetaInfo": 0
}
}