Mongo matching only where first element of array has certain field value - arrays

Question

I have a query below that extracts a couple of values from a large nested document. It tells me the user id and the first item name for each order.
This works fine, however I want it to only return the record where the first item's name is not null and is not blank. I can't figure out how to add a second query to the $match operator below to achieve this
db.getCollection('Orders').aggregate
([
{ $match : { "Items.1" : { $exists : true }}, ???},
{ $project: {
_id:0,
'UserId': '$User.EntityId',
'ItemName': {$arrayElemAt: ['$Items.Details.ItemName', 0]}
}
}
]);
Edited to show sample document
{
"_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

You need to add the two conditions to your existing $match (not null and not blank) to check the Items as:
$match : { "Items.1" : { $exists : true, "$ne": null,"$ne":""}
If you want to check the element Items[0].Details.ItemName you can doing using the operator $and
{ $match : {
$and: [
{"Items.1" : { $exists : true }},
{"Items.Details.ItemName" : { $ne : null}},
{"Items.Details.ItemName" : { $ne : ""}},
]
}},