mongo query: add a new field with ranking number based on another field - mongodb

Question

I am new to mongo queries. Currently I have a collection like this, which is used to create a d3 force-directed graph.
{
"_id": "allesgute3",
"nodes": [{
"id": "bmw#gmail.com",
"count": 15,
"nodeUpdatetime": 1525341732
}, {
"id": "abc#gmail.com",
"count": 10,
"nodeUpdatetime": null
}, {
"id": "xyz#gmail.com",
"count": 8,
"nodeUpdatetime": 1525408742
}, {
"id": "wilson#gmail.com",
"count": 4,
"nodeUpdatetime": 1525423847
}, {
"id": "niv#gmail.com",
"count": 6,
"nodeUpdatetime": 1525447758
}, {
"id": "car#gmail.com",
"count": 9,
"nodeUpdatetime": 1525447763
},
{
"id": "jason#gmail.com",
"count": 1,
"nodeUpdatetime": 1525447783
}
],
"links": [{
"source": "bmw#gmail.com",
"target": "jason#gmail.com",
"timestamp": 1525312111
}, {
"source": "car#gmail.com",
"target": "jason#gmail.com",
"timestamp": 1525334013
}, {
"source": "bmw#gmail.com",
"target": "car#gmail.com",
"timestamp": 1525334118
}]
}
Using a mongo query, I would like to generate the output to something like this. Basically for the nested data under "nodes", add a new field called "topn" and rank them by count from 1 to 5. The remainder values are null. Can anyone help? Thank you!
{
"_id": "allesgute3",
"nodes": [{
"id": "bmw#gmail.com",
"count": 15,
"nodeUpdatetime": 1525341732,
"topn": 1
}, {
"id": "abc#gmail.com",
"count": 10,
"nodeUpdatetime": null,
"topn": 2
}, {
"id": "xyz#gmail.com",
"count": 8,
"nodeUpdatetime": 1525408742,
"topn": 4
}, {
"id": "wilson#gmail.com",
"count": 4,
"nodeUpdatetime": 1525423847,
"topn": null
}, {
"id": "niv#gmail.com",
"count": 6,
"nodeUpdatetime": 1525447758,
"topn": 5
}, {
"id": "car#gmail.com",
"count": 9,
"nodeUpdatetime": 1525447763,
"topn": 3
},
..............

Answer 1

The following should get you what you want:
db.collection.aggregate({
$unwind: "$nodes" // flatten the "nodes" array
}, {
$sort: { "nodes.count": -1 } // sort descending by "count"
}, {
$group: { // create the original structure again - just with sorted array elements
_id: "$_id",
nodes: { "$push": "$nodes" }
}
}, {
$addFields: {
"nodes": {
$zip: { // zip two arrays together
inputs: [
"$nodes", // the first one being the existing and now sorted "nodes" array
{ $range: [ 1, 6 ] } // and the second one being [ 1, 2, 3, 4, 5 ]
],
useLongestLength: true // do not stop after five elements but instead continue using a "null" value
}
}
}
}, {
$addFields: {
"nodes": {
$map: { // transform the "nodes" array
input: "$nodes",
as: "this",
in: {
$mergeObjects: [ // by merging two objects
{ $arrayElemAt: [ "$$this", 0] }, // the first sits at array position 0
{
topn: { $arrayElemAt: [ "$$this", 1] } // the second will be a new entity witha a "topn" field holding the second element in the array
}
]
}
}
}
}
})