get sum of integer from array of objects in mongodb - mongodb

Question

I want to filter my documents by sum of decimal field in array of objects, but didn't find anything good enough. for example I have documents like below:
[
{
"id": 1,
"limit": NumberDecimal("100000"),
"requests": [
{
"money": NumberDecimal("50000"),
"user": "user1"
}
]
},
{
"id": 2,
"limit": NumberDecimal("100000"),
"requests": [
{
"money": NumberDecimal("100000"),
"user": "user2"
}
]
},
{
"id": 1,
"limit": null,
"requests": [
{
"money": NumberDecimal("50000"),
"user": "user1"
},
{
"money": NumberDecimal("50000"),
"user": "user3"
}
]
},
]
description by documents fields:
limit - maximum amount of money, that I have
requests - array of objects, where money it's how much money user get from limit (if user1 get 50000 money there remainder it's 50000, limit - sum(requests.money))
I am making query in mongodb from scala projects:
get all documents where limit equal to null
get all documents where I have x remainder money (x like input value)
first case it's more easy than second one, I know how I can get sum of requests.money: I am doing it by this query:
db.campaign.aggregate([
{$project: {
total: {$sum: ["$requests.money"]}
}}
])
scala filter part
Filters.or(
Filters.equal("limit", null),
Filters.expr(Document(s""" {$$project: {total: {$$sum: ["$$requests.money"]}}}"""))
)
But I don't want to store it and get as result, I want to filter by this condition x (money which I want to get by some user) limit >= sum(requests.money) + x. And by this filter I want to get all filtered documents.
Example:
x = 50000
and output must be like this:
[
{
"id": 1,
"limit": NumberDecimal("100000"),
"requests": [
{
"money": NumberDecimal("50000"),
"user": "user1"
}
]
},
{
"id": 1,
"limit": null,
"requests": [
{
"money": NumberDecimal("50000"),
"user": "user1"
},
{
"money": NumberDecimal("50000"),
"user": "user3"
}
]
},
]

Answer 1

You have to use an aggregation pipeline like this:
db.campaign.aggregate([
{
$set: {
remainder: {
$subtract: [ "$limit", { $sum: "$requests.money" } ]
}
}
},
{
"$match": {
$or: [
{ limit: null },
{ remainder: { $gte: 0 } }
]
}
},
{ $unset: "remainder" }
])
Mongo Playground
This one is also possible, but more difficult to read:
db.campaign.aggregate([
{
"$match": {
$or: [
{ limit: null },
{
$expr: {
$gt: [
{ $subtract: [ "$limit", { $sum: "$requests.money" } ] },
0
]
}
}
]
}
}
])