match element in the array with aggregation - mongodb

Question

i have mongo db collection the follwing structure
{
{
"_id" : ObjectId("63e37afe7a3453d5014c011b"),
"schemaVersion" : NumberInt(1),
"Id" : "ObjectId("63e37afe7a3453d5014c0112")",
"Id1" : "ObjectId("63e37afe7a3453d5014c0113")",
"Id2" : "ObjectId("63e37afe7a3453d5014c0114")",
"collectionName" : "Country",
"List" : [
{
"countryId" : NumberInt(1),
"name" : "Afghanistan",
},{
"countryId" : NumberInt(1),
"name" : "India",
},
{
"countryId" : NumberInt(1),
"name" : "USA",
}
}
i need to match the value with id, id1, id2, collectionName and name in the list to get country id for example if match the below value
"Id" : "ObjectId("63e37afe7a3453d5014c0112")",
"Id1" : "ObjectId("63e37afe7a3453d5014c0113")",
"Id2" : "ObjectId("63e37afe7a3453d5014c0114")",
"collectionName" : "Country",
"name" : "Afghanistan",
i need result
{
"countryId" : 1,
"name" : "Afghanistan",
}
i tried like below
db.country_admin.aggregate([
{ $match: { collectionName: "Country" } },
{ $unwind : '$countryList' },
{ $project : { _id : 0, 'countryList.name' : 1, 'countryList.countryId' : 1 } }
]).pretty()
and i have following output
[
{
"List" : {
"countryId" : 1.0,
"name" : "Afghanistan"
}
},
{
"List" : {
"countryId" : 2.0,
"name" : "india"
}
},
{
"List" : {
"countryId" : 3.0,
"name" : "USA"
}
}]```

Answer 1

You can try using $filter to avoid $unwind like this example:
First $match by your desired condition(s).
Then $filter and get the first element (as "List.name": "Afghanistan" is used into $match stage there will be at least one result).
And output only values you want using $project.
db.collection.aggregate([
{
"$match": {
"Id": ObjectId("63e37afe7a3453d5014c0112"),
"Id1": ObjectId("63e37afe7a3453d5014c0113"),
"Id2": ObjectId("63e37afe7a3453d5014c0114"),
"collectionName": "Country",
"List.name": "Afghanistan",
}
},
{
"$project": {
"country": {
"$arrayElemAt": [
{
"$filter": {
"input": "$List",
"cond": {
"$eq": [
"$$this.name",
"Afghanistan"
]
}
}
},
0
]
}
}
},
{
"$project": {
"_id": 0,
"countryId": "$country.countryId",
"name": "$country.name"
}
}
])
Example here
By the way, using $unwind is also possible and you can check this example