Mongo DB aggregation with $project and $filter: $add and $subtract return null - mongodb

Question

So I'm running a pretty big aggregation query in mongo shell (just for testing purpose)
in my last $project step, i use $filter to select a range of elements.
$filter: {
"input": "$users",
"as": "users",
"cond": {
$and: [
{
$lte: [
"$$users.ranking",
{$add: ["$myUser[0].ranking", 5]}
]
},
{
$gte: [
"$$users.ranking",
{$subtract: ["$myUser[0].ranking", 5]}
]
}
]
}
}
$subtract and $add both return null, any idea how i get it correct?
MongoVersion: 3.6.3, running in a docker container using the mongo 3.6.3 image.
Correct output should be:
"users" : [
{
"_id" : ObjectId("5ba3c2089a3a3e26a859f11b"),
"sgId" : ObjectId("5b76c1040c3aa5000559e6b3"),
"score" : 30,
"ranking" : NumberLong("0")
},
{
"_id" : ObjectId("5ba3c1d89a3a3e26a859f11a"),
"sgId" : ObjectId("5b76c1000c3aa500060e0fd2"),
"score" : 20,
"ranking" : NumberLong("1")
},
{
"_id" : ObjectId("5ba4fa3b71936b33e46569b9"),
"sgId" : ObjectId("5b76c8a3f7d606000566b652"),
"score" : 10,
"ranking" : NumberLong("2")
},
{
"_id" : ObjectId("5ba4fa4c71936b33e46569ba"),
"sgId" : ObjectId("5b76cafbf7d6060006270c90"),
"score" : 9,
"ranking" : NumberLong("3")
},
{
"_id" : ObjectId("5ba4fe6e71936b33e46569bb"),
"sgId" : ObjectId("5b7a4e69f7d606000566b65f"),
"score" : 8,
"ranking" : NumberLong("4")
},
{
"_id" : ObjectId("5ba4fe7471936b33e46569bc"),
"sgId" : ObjectId("5b7a4f47f7d6060006270cc4"),
"score" : 7,
"ranking" : NumberLong("5")
},
{
"_id" : ObjectId("5ba4fe8871936b33e46569bd"),
"sgId" : ObjectId("5b7a5265f7d606000566b67e"),
"score" : 6,
"ranking" : NumberLong("6")
}
]
Complete Query:
db.highscore.aggregate([
{
$sort: {score: -1}
},
{
$group: {
"_id": false,
"users": {
$push: {
"_id": "$_id",
"sgId": "$sgId",
"score": "$score",
}
}
}
},
{
$unwind: {
"path": "$users",
"includeArrayIndex": "ranking"
}
},
{
$group: {
"_id": false,
"users": {
$push: {
"_id": "$users._id",
"sgId": "$users.sgId",
"score": "$users.score",
"ranking": "$ranking"
}
}
}
},
{
$project: {
"users": "$users",
"myUser": {
$filter: {
"input": "$users",
"as": "user",
"cond": {
$eq: ["$$user.sgId", ObjectId("5b76c1000c3aa500060e0fd2")]
}
}
}
}
},
{
$project: {
"myUser": "$myUser",
"users" : {
$filter: {
"input": "$users",
"as": "users",
"cond": {
$and: [
{
$lte: [
"$$users.ranking",
{$add: ["$myUser[0].ranking", NumberLong("5")]}
]
},
{
$gte: [
"$$users.ranking",
{$subtract: ["$myUser[0].ranking", NumberLong("5")]}
]
}
]
}
}
}
}
},
])
Used Documents:
{
"_id" : ObjectId("5ba3c1d89a3a3e26a859f11a"),
"sgId" : ObjectId("5b76c1000c3aa500060e0fd2"),
"type" : "a",
"score" : 20,
"created" : ISODate("2018-09-20T17:50:48.024+02:00")
},
{
"_id" : ObjectId("5ba3c2089a3a3e26a859f11b"),
"sgId" : ObjectId("5b76c1040c3aa5000559e6b3"),
"type" : "a",
"score" : 30,
"created" : ISODate("2018-09-20T17:51:36.258+02:00")
},
{
"_id" : ObjectId("5ba4fa3b71936b33e46569b9"),
"sgId" : ObjectId("5b76c8a3f7d606000566b652"),
"type" : "a",
"score" : 10,
"created" : ISODate("2018-09-20T17:50:48.024+02:00")
},
{
"_id" : ObjectId("5ba4fa4c71936b33e46569ba"),
"sgId" : ObjectId("5b76cafbf7d6060006270c90"),
"type" : "a",
"score" : 9,
"created" : ISODate("2018-09-20T17:50:48.024+02:00")
}

Answer 1

Found it,
i just needed to add an $unwind before the last $project to convert the myUser Array into an object - then i was able to reach it for the add.
So full pipeline to get rankings of a highscore list and a range with your given user as source.
db.highscore.aggregate([
{
$sort: {score: -1}
},
{
$group: {
"_id": false,
"users": {
$push: {
"_id": "$_id",
"sgId": "$sgId",
"score": "$score",
}
}
}
},
{
$unwind: {
"path": "$users",
"includeArrayIndex": "ranking"
}
},
{
$group: {
"_id": false,
"users": {
$push: {
"_id": "$users._id",
"sgId": "$users.sgId",
"score": "$users.score",
"ranking": "$ranking"
}
}
}
},
{
$project: {
"users": "$users",
"myUser": {
$filter: {
"input": "$users",
"as": "user",
"cond": {
$eq: ["$$user.sgId", ObjectId("5b76c1000c3aa500060e0fd2")]
}
}
}
}
},
{
$unwind: {
path: '$myUser'
}
},
{
$project: {
"myUser": "$myUser",
"users" : {
$filter: {
"input": "$users",
"as": "users",
"cond": {
$and: [
{
$lte: [
"$$users.ranking",
{$add: ["$myUser.ranking", NumberLong("2")]}
]
},
{
$gte: [
"$$users.ranking",
{$subtract: ["$myUser.ranking", NumberLong("2")]}
]
}
]
}
}
}
}
},
], {'allowDiskUse': true})