clone and rename a field of an array of subdocuments in Mongo - mongodb

Question

I've got a collection like this:
{
name: "A Name",
answers: [
{order: 1},
{order: 2},
{order: 3}
]
}
What I want to do is to add a new filed id to each element of the answers array based on the value of the order property - I want just to clone it, so the output is
{
name: "A Name",
answers: [
{order: 1, id: 1},
{order: 2, id: 2},
{order: 3, id: 3}
]
}
I looked at this post and this one too, but I don't know how to combine them to work properly for subdocuments.
In MongoDB documentation for the aggregate method, I found a simple example of how to update embedded documents here, but I have no idea how to use the order property instead of a fixed term. The following tries seem not to work as I need:
db.collection.aggregate([
{
$addFields: {
"answers.id": "answers.$.order"
}
}
])
db.collection.aggregate([
{
$addFields: {
"answers.id": "$answers.order"
}
}
])
Is it possible to achieve the expected result with the `aggregate method?

Answer 1

Demo - https://mongoplayground.net/p/M79MV6-Zp4C
db.collection.aggregate([
{
$set: {
answers: {
$map: {
input: "$answers",
as: "answer",
in: { $mergeObjects: [ "$$answer", { id: "$$answer.order" } ]
}
}
}
}
}
])
Updated Demo - https://mongoplayground.net/p/iyqIPGQ5-ld
db.collection.aggregate([
{ $unwind: "$answers" },
{
$group: {
_id: "$_id",
answers: { $push: { order: "$answers.order", id: "$answers.order" } },
name: { $first: "$name" } // preserve properties add them to the group pick 1st value
}
}
])
Demo - https://mongoplayground.net/p/Ln5CcmT-Kkm
db.collection.aggregate([
{ $unwind: "$answers" }, // break into individuals documents
{ $addFields: { "answers.id": "$answers.order" } }, // copy order value to id
{ $group: { _id: "$_id", answers: { $push: "$answers" } } } // join and group it back
])
If you want to sort n get id from index
Demo - https://mongoplayground.net/p/6U_sRYWHtDR
db.collection.aggregate([
{ $sort: { "answers.order": 1 } },
{ $unwind: { path: "$answers", includeArrayIndex: "index" } },
{ $group: { _id: "$_id", answers: { "$push": { order: "$answers.order", id: { $add: [ "$index", 1 ] } } } } }
])