How to create objects inside Mongodb map - mongodb

Question

I have a document that looks like this, I am using mongo $map to project fields inside the table and rename the key. I can't use $unwind due to some internal complexity.
{
"Table":[
{"Lookup":{
"CreatedBy":{
"id": "User001",
"Name":"UserName"
}
}
}]
}
The output I am expecting looks something like this
{
"Table":[
{"Lookup":{
"CreatedBy":"UserName"
}
}]
}
I am trying to achieve it with mongo $map but it is not supported
db.getCollection('TableDoc').aggregate([
{
"$project": {
"Table": {
"$map": {
"input": "$Table",
"in": {
"Lookup.CreatedAt": "$$this.Lookup.CreatedAt.Name",
}
}
}
}
}
])
Is there any other way to achieve this without using $unwind

Answer 1

This is supported with $map, but just not using "dotted field paths". Instead you use "absolute" object structures:
collection.aggregate([
{ "$addFields": {
"Table": {
"$map": {
"input": "$Table",
"in": {
"Lookup": {
"CreatedBy": "$$this.Lookup.CreatedBy.Name"
}
}
}
}
}}
])
Alternately if you have lots of fields in the objects you can use $mergeObjects where supported:
collection.aggregate([
{ "$addFields": {
"Table": {
"$map": {
"input": "$Table",
"in": {
"$mergeObjects": [
"$$this",
{
"Lookup": {
"CreatedBy": "$$this.Lookup.CreatedBy.Name"
}
}
]
}
}
}
}}
])
That makes more sense when an example shows more fields than your sample in the question does.