MongoDB aggregations. Get value differences - mongodb

Question

I've been struggling with mongo trying to find a solution to show the differences between values.
I have values like this:
[
{val: 1},
{val: 4},
{val: 7},
{val: 8},
{val: 11}
]
And I want to receive something like this:
[
{diff: 3},
{diff: 3},
{diff: 1},
{diff: 3}
]
Every value is evaluated by taking the next one (4) and subtracting the previous one (1). After all this, we receive 3 in output, which is located in the second list as the first item.
Is it possible to achieve it using MongoDB aggregations?

Answer 1

You need to group them into array, calculate diff and flatten again.
Pseudocode
//We $group here all values
var _data = [{val: 1}, {val: 4}, ..., {val: 11}];
//With $range operator we get nº of items
// We ensure even items, since odd items will return null as last item
var idx = [0, 1, 2, ..., n];
//Here we store diff items with $map operator
var data = [];
//$map
for (var i in idx) {
data[i] = _data[i+1] - _data[i];
}
//$unwind
{data:[0]}, {data[1]}, {data[2]}, ...
//$replaceRoot
{
data:{ {
diff : 3 --> diff : 3
} }
}
Add these steps into your pipeline:
db.collection.aggregate([
{
$group: {
_id: null,
data: { $push: "$$ROOT" }
}
},
{
$addFields: {
data: {
$map: {
input: {
$range: [
0,
{
$subtract: [
{ $size: "$data" },
{ $mod: [ { $size: "$data" }, 2 ] }
]
},
1
]
},
as: "idx",
in: {
diff: {
$subtract: [
{
$arrayElemAt: [
"$data.val",
{
$add: [ "$$idx", 1 ]
}
]
},
{
$arrayElemAt: [ "$data.val", "$$idx" ]
}
]
}
}
}
}
}
},
{
$unwind: "$data"
},
{
$replaceRoot: {
newRoot: "$data"
}
}
])
MongoPlayground