Mongoose aggregate until sum of documents is equal to a certain value - mongodb

Question

What pipeline can i use to select all ids until i hit the sum of 180 and get the _ids. Below is a sample of the data that i've filtered out already. In this case it should select the first two items.
[
{
"_id": "6048b2b190422d0066d90740",
"Code": "A0ABI61YH",
"Amount": 100
},
{
"_id": "6048b3cc7e4b350072424f4c",
"Code": "A0ABEAXX6",
"Amount": 100
},
{
"_id": "6048b5167e4b350072424f50",
"Code": "A0ABCENPD",
"Amount": 100
}
]

Answer 1

I don't think is there any straight way to achieve this, if you really want to then try, this will only work when your data size below 16MB, because we are going to group your documents in a document in array, second this may cause the performance issues,
$group by null and group all documents and required fields (_id, Amount) in result
$reduce to iterate loop of result array,
initialValue declare initial value for Amount is 0 and result is []
in check condition if initialValue's Amount is less than 180 then concat current object's _id and initialValue's _ids using $concatArrays and sum current object's Amount and initialValue's Amount, otherwise return same value
db.collection.aggregate([
{
$group: {
_id: null,
result: {
$push: {
_id: "$_id",
Amount: "$Amount"
}
}
}
},
{
$project: {
_id: 0,
result: {
$reduce: {
input: "$result",
initialValue: { Amount: 0, _ids: [] },
in: {
$cond: [
{ $lt: ["$$value.Amount", 180] },
{
_ids: { $concatArrays: ["$$value._ids", ["$$this._id"]] },
Amount: { $sum: ["$$value.Amount", "$$this.Amount"] }
},
"$$value"
]
}
}
}
}
}
])
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