Mongo aggregation, project a subfield of the first element in the array - mongodb

Question

I have a sub collection of elements and I want to project a certain subfield of the FIRST item in this collection. I have the following but it only projects the field for ALL elements in the array.
Items is the subcollection of Orders and each Item object has a Details sub object and an ItemName below that. I want to only return the item name of the FIRST item in the list. This returns the item name of every item in the list.
How can I tweak it?
db.getCollection('Orders').aggregate
([
{ $match : { "Instructions.1" : { $exists : true }}},
{ $project: {
_id:0,
'UserId': '$User.EntityId',
'ItemName': '$Items.Details.ItemName'
}
}
]);
Updated:
{
"_id" : "order-666156",
"State" : "ValidationFailed",
"LastUpdated" : {
"DateTime" : ISODate("2017-09-26T08:54:16.241Z"),
"Ticks" : NumberLong(636420128562417375)
},
"SourceOrderId" : "666156",
"User" : {
"EntityId" : NumberLong(34450),
"Name" : "Bill Baker",
"Country" : "United States",
"Region" : "North America",
"CountryISOCode" : "US",
},
"Region" : null,
"Currency" : null,
"Items" : [
{
"ClientOrderId" : "18740113",
"OrigClientOrderId" : "18740113",
"Quantity" : NumberDecimal("7487.0"),
"TransactDateTime" : {
"DateTime" : Date(-62135596800000),
"Ticks" : NumberLong(0)
},
"Text" : null,
"LocateRequired" : false,
"Details" : {
"ItemName" : "Test Item 1",
"ItemCost" : 1495.20
}
},
{
"ClientOrderId" : "18740116",
"OrigClientOrderId" : "18740116",
"Quantity" : NumberDecimal("241.0"),
"TransactDateTime" : {
"DateTime" : Date(-62135596800000),
"Ticks" : NumberLong(0)
},
"Text" : null,
"LocateRequired" : false,
"Details" : {
"ItemName" : "Test Item 2",
"ItemCost" : 2152.64
}
}
]
}

Answer 1

In case your are using at least MongoDB v3.2 you can use the $arrayElemAt operator for that. The below query does what you want. It will, however, not return any data for the sample you provided because the "Instructions.1": { $exists: true } filter removes the sample document.
db.getCollection('Orders').aggregate([{
$match: {
"Instructions.1": {
$exists: true
}
}
}, {
$project: {
"_id": 0,
"UserId": "$User.EntityId",
"ItemName": { $arrayElemAt: [ "$Items.Details.ItemName", 0 /* first item! */] }
}
}])