How to project single array field - mongodb

Question

starting from this document schema
{
"_id" : ObjectId("5a5cfde58c8a4a35a89726b4"),
"Names" : [
{ "Code" : "en", "Name" : "ITALY" },
{ "Code" : "it", "Name" : "ITALIA" }
],
"TenantID" : ObjectId("5a5cfde58c8a4a35a89726b2"),
...extra irrelevant fields
}
I need to get an object like this
{
"ID" : ObjectId("5a5cfde58c8a4a35a89726b4"),
"Name" : "ITALY"
}
Filtering by array's Code field (in the sample by 'en').
I wrote an aggregate query, this
db.Countries.aggregate([{$project: {_id:0, 'ID': '$_id', 'Names': {$filter: {input: '$Names', as: 'item', cond: {$eq: ['$$item.Code', 'en']}}}}},{$skip: 10},{$limit: 5}]);
that correctly return only documents with 'en' Names values, returning only the sub array matching elements.
Now I cannot find the way to return only the Name field value into a new 'Name' field.
Which is the best and most performing method to do this ?
Actually I'm trying on Mongo shell, but later I need to replicate this behavior with .NET driver.
Using MongoDB 3.6
Thanks

Answer 1

You can try below aggregation query.
$match to only consider documents where there is atleast one element in array matching input criteria.
$filter array on the input criteria and $arrayElemAt to project the single matched element.
$let to output name from matched object.
db.Countries.aggregate([
{"$match":{"Names.Code":"en"}},
{"$project":{
"_id":0,
"ID": "$_id",
"Name":{
"$let":{
"vars":{
"obj":{
"$arrayElemAt":[
{"$filter":{
"input":"$Names",
"as":"name",
"cond":{"$eq":["$$name.Code","en"]}
}},
0]
}
},
"in":"$$obj.Name"
}
}
}},
{"$skip": 10},
{"$limit": 5}
])