Mongodb $subtract document value from the next one - mongodb

Question

I have such a collection of documents:
{ _id: 1, Meters: { gasmeter: 1000.0 } }
{ _id: 2, Meters: { gasmeter: 1007.0 } }
{ _id: 3, Meters: { gasmeter: 1010.0 } }
And I am trying to get the difference between the gasmeter elements as such:
{ difference: 7 } // Difference between _id=1 and _id=2
{ difference: 3 } // Difference between _id=2 and _id=3
I've tried:
db.MeterData.aggregate([{$project: { item: 1, difference: {$subtract: [ {$add: [ "$Meters.gasmeter","$Meters.gasmeter" ] }, "$Meters.gasmeter" ] }}}])
But this does not work. I still get the same values.
Any idea how to do that aggregation with mongodb?

Answer 1

Since I'm not entirely sure how you wish to solve the problem (in C# or mongo itself). I've come up with this in Mongo.
db.MeterData.find().forEach(
function (doc) {
var next = db.Sum.findOne({
_id: {
"$gt": NumberInt(doc._id)
}
})
if (next != null) {
var diff = next.Meters.gasmeter - doc.Meters.gasmeter;
printjson("{ difference : " + diff + "} - Difference between Id=" + doc._id + " and Id=" + next._id);
}
})
Which returns
"{ difference : 7} - Difference between Id=1 and Id=2"
"{ difference : 3} - Difference between Id=2 and Id=3"
I hope this helps you a bit.

Answer 2

Starting in Mongo 5, it's a perfect use case for the new $setWindowFields aggregation operator:
// { id: 1, gasmeter: 1000 }
// { id: 2, gasmeter: 1007 }
// { id: 3, gasmeter: 1010 }
db.collection.aggregate([
{ $setWindowFields: {
sortBy: { id: 1 },
output: {
difference: {
$push: "$gasmeter",
window: { range: [-1, "current"] }
}
}
}},
// { id: 1, gasmeter: 1000, difference: [1000] }
// { id: 2, gasmeter: 1007, difference: [1000, 1007] }
// { id: 3, gasmeter: 1010, difference: [1007, 1010] }
{ $match: { $expr: { $eq: [{ $size: "$difference" }, 2] } } },
// { id: 2, gasmeter: 1007, difference: [1000, 1007] }
// { id: 3, gasmeter: 1010, difference: [1007, 1010] }
{ $set: {
difference: { $subtract: [{ $last: "$difference" }, { $first: "$difference" }] }
}}
])
// { id: 2, gasmeter: 1007, difference: 7 }
// { id: 3, gasmeter: 1010, difference: 3 }
This:
starts with a $setWindowFields aggregation stage which adds the difference field in each document (output: { difference: { ... }})
as an array $push of gasmeters ($sum: "$gasmeter")
from the specified span of documents (the window) which here is the "current" document and the previous one "-1": window: { range: [-1, "current"] }
then filters out the first document that doesn't have a previous document (i.e. we can't diff its value with a previous document's value):
to do so we simply check that the size of the array produced with the previous stage is 2: { $match: { $expr: { $eq: [{ $size: "$difference" }, 2] } } },
and finally substracts the 2 values in the array to get the actual difference: $subtract: [{ $last: "$difference" }, { $first: "$difference" }]