Mongodb use $elemMatch into $filter - arrays

Question

I have a document like this:
I need to return the documents and filter the nested array (lessons) where any item of subLessons into input array
[{
"_id": {
"$oid": "6081fedbee5d133dbffb42eb"
},
"name": "my quiz",
"lessons": [
{
"_id": "460c42e1-b0b7-437e-ab63-c59cce8ced0d",
"name": "section",
"subLesson": [
{
"$oid": "6081fed9ee5d133dbffb3cba"
},
{
"$oid": "6081fed9ee5d133dbffb3cc0"
}
]
},
{
"_id": "f7b5c95f-1a68-42ca-880c-22ef3831ff03",
"name": "ffff",
"subLesson": [
{
"$oid": "6081fed9ee5d133dbffb3cbb"
}
]
}
]
}
}]
I wrote the following query but it does not work. I do not know how to use $elemMatch in $filter
db.collection.aggregate([
{
"$project": {
_id: 1,
lessons: {
$filter: {
"input": "$lessons",
"as": "lesson",
"cond": {
"$$lesson.subLesson": {
"$elemMatch": {
"$in": [
ObjectId("6081fed9ee5d133dbffb3cba")
]
}
}
}
}
}
}
}
])
I am trying to find the record such that the result looks like the following.
[{
"_id": {
"$oid": "6081fedbee5d133dbffb42eb"
},
"lessons": [
{
"_id": "460c42e1-b0b7-437e-ab63-c59cce8ced0d",
"name": "zzzz",
"subLesson": [
{
"$oid": "6081fed9ee5d133dbffb3cba"
},
{
"$oid": "6081fed9ee5d133dbffb3cc0"
}
]
}
]
},
}]
Can anyone please help out to understand how can I make this work
thanks

Answer 1

You can use $in directly
db.collection.aggregate([
{
$project: {
lessons: {
$filter: {
input: "$lessons",
cond: {
$in: [ ObjectId("6081fed9ee5d133dbffb3cba"), "$$this.subLesson" ]
}
}
}
}
}
])
Working Mongo playground
Update 1
db.collection.aggregate([
{ "$unwind": "$lessons" },
{
"$match": {
"lessons.subLesson": {
$in: [ ObjectId("6081fed9ee5d133dbffb3cba"), ObjectId("6081fed9ee5d133dbffb3cbb") ]
}
}
},
{
$group: {
_id: "$_id",
name: { $first: "$name" },
lessons: { $push: "$lessons" }
}
}
])
Mongo Playground