Is there a way to give order field to the result of MongoDB aggregation? - mongodb

Question

Is there any way to give order or rankings to MongoDB aggregation results?
My result is:
{
"score":100
"name": "John"
},
{
"score":80
"name": "Jane"
},
{
"score":60
"name": "Lee"
}
My wanted result is:
{
"score":100
"name": "John",
"rank": 1
},
{
"score":80
"name": "Jane"
"rank": 2
},
{
"score":60
"name": "Lee"
"rank": 3
}
I know there is a operator called $includeArrayIndex but this only works with $unwind operator.
Is there any way to give rank without using $unwind?
Using $unwind requires grouping on my collection, and I'm afraid grouping pipeline would be too huge to process.

Answer 1

The other way is to use $map and add rank in document using its index, and don't use $unwind stage because it would be single field array you can directly access using its key name as mention in last line of code,
$group by null and make array of documents in root array,
$map to iterate loop of root array, get the index of current object from root array using $indexOfArray and increment that returned index number using $add because index start from 0, and that is how we are creating rank field, merge object with current element object and rank field using $mergeObjects
let result = await db.collection.aggregate([
{
$group: {
_id: null,
root: {
$push: "$$ROOT"
}
}
},
{
$project: {
_id: 0,
root: {
$map: {
input: "$root",
in: {
$mergeObjects: [
"$$this",
{
rank: { $add: [{ $indexOfArray: ["$root", "$$this"] }, 1] }
}
]
}
}
}
}
}
]);
// you can access result using root key
let finalResult = result[0]['root'];
Playground