Mongodb $ (dollar sign) project query for arrays - arrays

Question

I have the following data:
{
_id: "1"
transitions: [
{
"_id" : "11"
"name" : "Tr1"
"checkLists" : [
{ _id: "111", name: "N1"},
{ _id: "112", name: "N2"}
]
}
]
}
I used the following code to get the name N2 by query of _id:112
db.collection.findOne({ 'transitions.checkLists._id: new ObjectId("112") } }}, { 'transitions.checkLists.$': 1 })
but the result returns back both of them:
{ _id: ObjectId("1"),
transitions:
[ { checkLists:
[ { name: 'N1', _id: ObjectId("111") },
{ name: 'N2', _id: ObjectId("112") } ] } ] }
I would like to find and get only the name N2 by query of _id:112
Expected Result:
{ _id: ObjectId("1"),
transitions:
[ { checkLists:
[ { name: 'N2', _id: ObjectId("112") } ] } ] }

Answer 1

You can do it via the aggregation framework as follow:
db.collection.aggregate([
{
$match: {
"transitions.checkLists._id": "111"
}
},
{
"$addFields": {
"transitions": {
"$map": {
"input": "$transitions",
"as": "t",
"in": {
"$mergeObjects": [
"$$t",
{
"checkLists": {
"$filter": {
"input": "$$t.checkLists",
"as": "c",
"cond": {
$eq: [
"$$c._id",
"111"
]
}
}
}
}
]
}
}
}
}
},
{
"$addFields": {
transitions: {
$filter: {
input: "$transitions",
as: "elem",
cond: {
"$ne": [
"$$elem.checkLists",
[]
]
}
}
}
}
}
])
Explained:
Match the transitions.checkLists._id element in first stage.
Map/mergeobjects with filtered checklists to filter only the needed object from the transitions.checkLists array.
Remove the transitions elements where no checkList exists.
( this need to be done to remove elements for same document where there is no matching _id's )
Playground