Is there a way in mongodb to group at multiple levels - mongodb

Question

I have a document which contains an array of array as given below.
This is the first document.
{
"_id": "5d932a2178fdfc4dc41d75da",
"data": [
{
"nestedData": [
{
"_id": "5d932a2178fdfc4dc41d75e1",
"name": "Special 1"
},
{
"_id": "5d932a2178fdfc4dc41d75e0",
"name": "Special 2"
}
]
}
]
}
I need to lookup(join) to another collection with the _id in the nestedData array in the aggregation framework.
The 2nd document from which I need to lookup is
{
"_id": "5d8b1ac3b15bc72d154408e1",
"status": "COMPLETED",
"rating": 4
}
I know I need to $unwind it twice to convert nestedData array into object.
But how do I group back again to form the same object like given below
{
"_id": "5d932a2178fdfc4dc41d75da",
"data": [
{
"array": [
{
"_id": "5d932a2178fdfc4dc41d75e1",
"name": "Special 1",
"data": {
"_id": "5d8b1ac3b15bc72d154408e1",
"status": "COMPLETED",
"rating": 4
},
{
"_id": "5d932a2178fdfc4dc41d75e0",
"name": "Special 2",
"data": {
"_id": "5d8b1ac3b15bc72d154408e0",
"status": "COMPLETED",
"rating": 4
},
}
]
}
]
}

Answer 1

Try this query
db.testers.aggregate([
{$lookup: {
from: 'demo2',
pipeline: [
{ $sort: {'_id': 1}},
],
as: 'pointValue',
}},
{
$addFields:{
"data":{
$map:{
"input":"$data",
"as":"doc",
"in":{
$mergeObjects:[
"$$doc",
{
"nestedData":{
$map:{
"input":"$$doc.nestedData",
"as":"nestedData",
"in":{
$mergeObjects:[
{ $arrayElemAt: [ {
"$map": {
"input": {
"$filter": {
"input": "$pointValue",
"as": "sn",
"cond": {
"$and": [
{ "$eq": [ "$$sn._id", "$$nestedData._id" ] },
]
}
}
},"as": "data",
"in": {
"name": "$$nestedData.name",
"data":"$$data",
}}
}, 0 ] },'$$nestedData'
],
}
}
}
}
]
}
}
}
}
},
{$project: { pointValue: 0 } }
]).pretty()