Find subdocument nested inside a document by id in mondogb - mongodb

Question

I have a mongodb document like this
{
"_id": {
"$oid": "6241dd90891458501c17d627"
},
"A": [
{
"_id": {
"$oid": "6241ddb1891458501c17d63e"
},
"B": [
{
"_id": {
"$oid": "6241ddc4891458501c17d674"
}
},
{
"_id": {
"$oid": "6241ddda891458501c17d675"
}
}
]
},
{
"_id": {
"$oid": "6241ddbe891458501c17d63f"
},
"B": [
{
"_id": {
"$oid": "6241ddda891458501c17d678"
}
},
{
"_id": {
"$oid": "6241ddda891458501c17d679"
}
}
]
}
]
}
This document has 2 nested arrays: an array of "A" elements, inside each element of "A" there's an array of "B" elements. I need to search by an _id of a "B" element, let's say 6241ddda891458501c17d679. I need a way to obtain this structure in mongodb
{
"_id": {
"$oid": "6241dd90891458501c17d627"
},
"A": [
{
"_id": {
"$oid": "6241ddbe891458501c17d63f"
},
"B": [
{
"_id": {
"$oid": "6241ddda891458501c17d679"
}
}
]
}
]
}
How can I achieve this? Thanks very much

Answer 1

Maybe something like this:
Option 1, Find:
db.collection.find({
"A.B._id": {
"$oid": "6241ddda891458501c17d679"
}
},
{
"A": {
"$filter": {
"input": {
"$map": {
"input": "$A",
"as": "a",
"in": {
"_id": "$$a._id",
"B": {
"$filter": {
"input": "$$a.B",
"as": "b",
"cond": {
"$eq": [
{
"$oid": "6241ddda891458501c17d679"
},
"$$b._id"
]
}
}
}
}
}
},
"as": "an",
"cond": {
"$ne": [
"$$an.B",
[]
]
}
}
}
})
Explained:
Use find() with match query on "A.B._id" ( good to have index on this filed for best performance)
In the filter part add $filter/map/filter combination to filter only the matching _id for array B elements and preserve the array A _id , also in the initial filter condition use only non-empty arrays [] to avoid having elements from empty arrays in the final result.
playground1
Option 2 , aggregation:
db.collection.aggregate([
{
$match: {
"A.B._id": {
"$oid": "6241ddda891458501c17d679"
}
}
},
{
"$addFields": {
"A": {
"$filter": {
"input": {
"$map": {
"input": "$A",
"as": "a",
"in": {
"_id": "$$a._id",
"B": {
"$filter": {
"input": "$$a.B",
"as": "b",
"cond": {
"$eq": [
{
"$oid": "6241ddda891458501c17d679"
},
"$$b._id"
]
}
}
}
}
}
},
"as": "an",
"cond": {
"$ne": [
"$$an.B",
[]
]
}
}
}
}
}
])
playground2