MongoDB - Calculate field based on previous item - mongodb

Question

I have a tricky scenario - I need to calculate extra field, based on value from previous field. I have no idea how to do it in performant manner. Any thoughts?
Data:
{
_id: 1,
score: 66,
created_at: "2021-04-01"
},
{
_id: 2,
score: 12,
created_at: "2021-04-03"
},
{
_id: 3,
score: 7,
created_at: "2021-04-06"
}
What I want to achieve
{
_id: 1,
score: 66,
total_score: 66 // The most oldest item, so total score is the same as current score
created_at: "2021-04-01"
},
{
_id: 2,
score: 12,
total_score: 78 // Sum of current score and previous total_score
created_at: "2021-04-03"
},
{
_id: 3,
score: 7,
total_score: 85 // Sum of current score and previous total_score
created_at: "2021-04-06"
}
Any insights appreciated.

Answer 1

You can try aggregation query,
$lookup with pipeline and match greater than query by _id to select the previous record
$group by null and get the sum of score
$arrayElemAt to get first element from lookup result
$ifNull to check if the result is null then return 0 otherwise return the current value
$add to get total of score and return score from lookup
db.collection.aggregate([
{
$lookup: {
from: "collection",
let: { id: "$_id" },
pipeline: [
{ $match: { $expr: { $gt: ["$$id", "$_id"] } } },
{
$group: {
_id: null,
score: { $sum: "$score" }
}
}
],
as: "total_score"
}
},
{
$addFields: {
total_score: {
$add: [
"$score",
{
$ifNull: [
{ $arrayElemAt: ["$total_score.score", 0] },
0
]
}
]
}
}
}
])
Playground