Not able to access object value by dynamic key in mongoDB - mongodb

Question

I have two collections i.e parent and chilednodes.
parent collection :
{
"_id" : "5e6c70e8996ddf1c28e14504",
"startDate" : "2020-02-25T14:01:58.697Z",
"active_id" : "child_vesrion_1",
"child_id" : "5e5e2cd4e972a95b6c32b5bf30"
}
chilednodes:
{
"_id" : "5e5e2cd4e972a95b6c32b5bf30",
"startDate" : "2020-02-25T14:01:58.697Z",
"endDate" : null,
"child_vesrion_1" : {
"childName" : "xyz",
"createdDate" : "2020-02-25T14:01:58.697Z",
"text" : "sometext",
"type" : "notype"
},
"child_vesrion_2" : {
"childName" : "abc",
"createdDate" : "2020-02-25T14:01:58.697Z",
"text" : "sometext2",
"type" : "notype"
},
"active" : "child_vesrion_1"
}
I am trying to get parent detail along with active child version detail. Here i am using aggregation and lookup.
I am constructing "finalList" field by using mongoDB $addFields.
Here is my query;
db.parent
.aggregate([
{
$match: {
'_id' : '5e6c70e8996ddf1c28e14504'
},
},
{
$lookup: {
from: 'chilednodes',
localField: 'child_id',
foreignField: '_id',
as: 'child',
},
},
{
$addFields: {
finalList: {
$map: {
input: '$child',
as: 'c',
in: {
$let: {
vars: { "currentVersionKey": "child_vesrion_1"},
in: {
child_id:'$$c._id',
start_date : '$$c.startDate',
current_version_Key : '$$currentVersionKey',
active_child_name : '$$c.currentVersionKey.childName'
}
}
},
},
},
},
},
{
$project: {
child: 0,
},
},
])
In the response (finalList) i need active child name. I am taking a variable "currentVersionKey" and assigning value as "child_vesrion_1" and using that variable to find the value in "chilednodes". I am not able to get active childname.
NOTE : For testing i have hardcoded "currentVersionKey". But it will be populated dynamically
Here is my expected result;
{
"_id" : "5e6c70e8996ddf1c28e14504",
"startDate" : "2020-02-25T14:01:58.697Z",
"active_id" : "child_vesrion_1",
"child_id" : "5e5e2cd4e972a95b6c32b5bf30",
"finalList" : [
{
"child_id" : "5e5e2cd4e972a95b6c32b5bf30",
"start_date" : "2020-02-25T14:01:58.697Z",
"current_version_Key" : "child_vesrion_1",
"active_child_name" : "xyz"
}
]
}
But i am not getting "active_child_name" : "xyz" in the response. Any helps on this would be appreciated.

Answer 1

You can use $lookup pipeline,
db.parent.aggregate([
{ $match: { "_id": "5e6c70e8996ddf1c28e14504" } },
{
$lookup: {
from: "childnodes",
let: { "child_id": "$child_id", "activeid": "$active_id" },
pipeline: [
{ $match: { "$expr": { $eq: ["$_id", "$$child_id"] } } },
{
$project: {
"child_id": "$_id",
"start_date": "$startDate",
"current_version_Key": "$active",
"active_child_name": {
"$reduce": {
"input": { "$objectToArray": "$$ROOT" },
"initialValue": "",
"in": {
"$cond": [{ "$eq": ["$$this.k", "$$activeid"] },
"$$this.v.childName",
"$$value"
]
}
}
}
}
}
],
as: "finalList",
},
}
])