$concat field with index in $map mongodb? [duplicate] - node.js

Question

This question already has answers here:
Add some kind of row number to a mongodb aggregate command / pipeline
(3 answers)
Closed 4 years ago.
I have following collection
{
"_id" : ObjectId("5b16405a8832711234bcfae7"),
"createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
"firstName": "Bruce",
"lastName": "Wayne"
},
{
"_id" : ObjectId("5b16405a8832711234bcfae8"),
"createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
"firstName": "Clerk",
"lastName": "Kent"
},
{
"_id" : ObjectId("5b16405a8832711234bcfae9"),
"createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
"firstName": "Peter",
"lastName": "Parker"
}
I need to $project one more key index with $concat with 'INV-00' + index of the root element
My output should be something like that
{
"_id" : ObjectId("5b16405a8832711234bcfae7"),
"createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
"firstName": "Bruce",
"lastName": "Wayne",
"index": "INV-001"
},
{
"_id" : ObjectId("5b16405a8832711234bcfae8"),
"createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
"firstName": "Clerk",
"lastName": "Kent",
"index": "INV-002"
},
{
"_id" : ObjectId("5b16405a8832711234bcfae9"),
"createdAt" : ISODate("2018-06-05T07:48:45.248Z"),
"firstName": "Peter",
"lastName": "Parker",
"index": "INV-003"
}
and can I change createdAt format to this Thu Jan 18 2018 using $dateToString or something else???
Thanks in advance!!!

Answer 1

While I would certainly recommend you to do that on the client side as opposed to inside MongoDB, here is how you could get what you want - pretty brute-force but working:
db.collection.aggregate([
// you should add a $sort stage here to make sure you get the right indexes
{
$group: {
_id: null, // group all documents into the same bucket
docs: { $push: "$$ROOT" } // just to create an array of all documents
}
}, {
$project: {
docs: { // transform the "docs" field
$map: { // into something
input: { $range: [ 0, { $size: "$docs" } ] }, // an array from 0 to n - 1 where n is the number of documents
as: "this", // which shall be accessible using "$$this"
in: {
$mergeObjects: [ // we join two documents
{ $arrayElemAt: [ "$docs", "$$this" ] }, // one is the nth document in our "docs" array
{ "index": { $concat: [ 'INV-00', { $substr: [ { $add: [ "$$this", 1 ] }, 0, -1 ] } ] } } // and the second document is the one with our "index" field
]
}
}
}
}
}, {
$unwind: "$docs" // flatten the result structure
}, {
$replaceRoot: {
newRoot: "$docs" // restore the original document structure
}
}])