Sum time series with different time stamps MongoDB - mongodb

Question

I have a mongoDB database with multiple time series data and each time stamp is a separate document with some additional meta data from sensors. I want to sum the two time series in an aggregation but I am heavily struggling with that and can't find any examples.
Assume we have sensor A and B and the time stamps from the different sensors don't align. See an example of the data below. Next I want to sum the "volume" metric of the two time series. So for the example below sensor A has two time stamps en sensor B 3. So the sum of A and B should have 5 time stamps such that the sum reflects all the changes in the total volume (see also the schematic example below).
Anyone knows how to solve this in a mongoDB aggregation query? I can only use the mongoDB query language and not use NodeJS.
Sensor A
{
"_id":5d67d9ee074e99274eef30d5
"sensor": A
"volume":12.4
"temperatue": 20
"timestamp":2019-08-29 15:58:06.093
"__v":0
}
{
"_id":5d67da66074e99274eef30ea
"sensor": A
"volume":12.3
"temperatue": 21
"timestamp":2019-08-29 16:48:06.078
"__v":0
}
..etc
Sensor B
{
"_id":5d67d9ee074e99274eef30d5
"sensor": B
"volume":32.4
"temperatue": 20
"timestamp":2019-08-29 15:55:06.093
"__v":0
}
{
"_id":5d67da66074e99274eef30ea
"sensor": B
"volume":21.2
"temperatue": 21
"timestamp":2019-08-29 16:49:06.178
"__v":0
}
{
"_id":5d67da66074e99274eef30ea
"sensor": B
"volume":22.3
"temperatue": 22
"timestamp":2019-08-29 17:04:06.078
"__v":0
}
..etc
Here also a sketch of the result I would like to have.

Answer 1

Try this one:
db.collection.aggregate([
// Determine start and end-time
{ $sort: { timestamp: -1 } },
{ $group: { _id: "$sensor", data: { $push: "$$ROOT" } } },
{
$set: {
data: {
$reduce: {
input: "$data",
initialValue: [],
in: {
$concatArrays: [
"$$value",
[
{
$mergeObjects: [
"$$this",
{
timestamp_end: {
$ifNull: [ { $last: "$$value.timestamp" }, "$$NOW" ]
}
}
]
}
]
]
}
}
}
}
},
{ $unwind: "$data" },
// find data per interval
{ $sort: { "data.timestamp": 1 } },
{
$group: {
_id: null,
data: { $push: "$data" },
timestamp: { $addToSet: "$data.timestamp" }
}
},
{
$set: {
sum_data: {
$map: {
input: "$timestamp",
as: "t",
in: {
$filter: {
input: "$data",
cond: {
$and: [
{ $lte: [ "$$this.timestamp", "$$t" ] },
{ $gt: [ "$$this.timestamp_end", "$$t" ] }
]
}
}
}
}
}
}
},
// sum up temperatures
{
$set: {
volume: {
$map: {
input: "$sum_data",
in: { $sum: "$$this.volume" }
}
},
result: { $range: [ 0, { $size: "$timestamp" } ] }
}
},
// Join arrays to final result
{
$project: {
result: {
$map: {
input: "$result",
as: "i",
in: {
timestamp: { $arrayElemAt: [ "$timestamp", "$$i" ] },
volume: { $arrayElemAt: [ "$volume", "$$i" ] }
}
}
}
}
}
])
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