mongodb - find previous and next document in aggregation framework - mongodb

Question

After applying a long pipeline to my collection I can obtain something like this:
{
{
"_id": "main1",
"title": "First",
"code": "C1",
"subDoc": {
"active": true,
"sub_id": "main1sub1",
"order": 1
}
},
{
"_id": "main2",
"title": "Second",
"code": "C2",
"subDoc": {
"active": true,
"sub_id": "main2sub1",
"order": 1
}
},
{
"_id": "main3",
"title": "Third",
"code": "C3",
"subDoc": {
"active": false,
"sub_id": "main3sub1",
"order": 1
}
}
}
The documents are already in the correct order. Now I have to find the document immediately preceding or following the one corresponding to a given parameter. For example, if I know { "code" : "C2" } I have to retrieve the previous document (example document with "code" : "C1").
I only need to get that document, not the others.
I know how to do it using the find () method and applying sort () and limit () in sequence, but I want to get the document directly in the aggregation pipeline, adding the necessary stages to do it.
I've tried some combinations of $ indexOfArray and $ arrayElemAt, but the first problem I encounter is that I don't have an array, it's just documents.
The second problem is that the parameter I know might sometimes be inside the subdocument, for example {"sub_id": "main3sub1"}, and again I should always get the previous or next parent document as a response (in the example, the pipeline should return document "main2" as previous document)
I inserted the collection in mongoplayground to be able to perform the tests quickly:
mongoplayground
Any idea?

Answer 1

If you want to retrieve only the previous document, use the following query:
First Approach:
Using $match,$sort,$limit
db.collection.aggregate([
{
$match: {
code: {
"$lt": "C2"
}
}
},
{
"$sort": {
code: -1
}
},
{
$limit: 1
}
])
MongoDB Playground
Second Approach:
As specified by # Wernfried Domscheit,
Converting to array and then using $arrayElemAt
db.collection.aggregate([
{
$group: {
_id: null,
data: {
$push: "$$ROOT"
}
}
},
{
$addFields: {
"index": {
$subtract: [
{
$indexOfArray: [
"$data.code",
"C2"
]
},
1
]
}
}
},
{
$project: {
_id: 0,
data: {
$arrayElemAt: [
"$data",
"$index"
]
}
}
},
{
$replaceRoot: {
newRoot: "$data"
}
}
])
MongoDB Playground