MongoDB $elemMatch display issue - mongodb

Question

Listed are the following sample documents in a test collection
My requirement is to extract *only" two fields
"host.name" and "host.config.storageDevice.scsiLun.lunType" matching the condition that "host.config.storageDevice.scsiLun.lunType" : "cdrom1" and "host.name" : "on-xxx"
Like I mentioned above , I only want attribute "lunType" to be displayed in the array and not "a" or "b"
I attempted to use both $elemMatch and $projection and it always seems to return all the attributes of array "lunType" ... Am I missing anything here
Attempted in reference to documentation
http://docs.mongodb.org/manual/reference/projection/elemMatch/
Query
db.test.find({"host.config.storageDevice.scsiLun": { $elemMatch: { "lunType" : "cdrom1" } } },{ "host.name" : 1, "host.config.storageDevice.scsiLun.$" : 1})
db.test.find({"host.config.storageDevice.scsiLun.lunType" : "cdrom1" },{ "host.name" : 1, "host.config.storageDevice.scsiLun.$" : 1})
Documents in collection
{
"_id" : ObjectId("51d57f3ad4ebc6c87962d4c0"),
"host" : {
"name" : "on-xxx",
"config" : {
"storageDevice" : {
"scsiLun" : [
{
"a" : "1",
"lunType" : "cdrom1"
},
{
"a" : "2",
"lunType" : "disk2"
},
{
"a" : "3",
"lunType" : "disk3"
}
]
}
}
}
}
,
{
"_id" : ObjectId("51d57f59d4ebc6c87962d4c1"),
"host" : {
"name" : "on-yyy",
"config" : {
"storageDevice" : {
"scsiLun" : [
{
"a" : "4",
"lunType" : "cdrom4"
},
{
"a" : "5",
"lunType" : "disk5"
},
{
"a" : "6",
"lunType" : "disk6"
}
]
}
}
}
}
,
{
"_id" : ObjectId("51d57f74d4ebc6c87962d4c2"),
"host" : {
"name" : "on-zzz",
"config" : {
"storageDevice" : {
"scsiLun" : [
{
"a" : "7",
"lunType" : "cdrom11"
},
{
"a" : "8",
"lunType" : "disk22"
},
{
"a" : "9",
"lunType" : "disk32"
}
]
}
}
}
}

Answer 1

All $elemMatch does is to return the first element of the array that matches a criteria, but it will give you the entire element, that is both "a" and "lunType" properties.
What might get the desired result with aggregation using $unwind to break down the array, then $match to filter and $project to show only the "lunType" field.
I didn't test the query but it should look like this:
db.test.aggregate(
{ $unwind : "$host.config.storageDevice.scsiLun" },
{ $match : { host.name : "on-xxx" ,
host.config.storageDevice.scsiLun.lunType : "cdrom1" } },
{ $project : {
_id : 0 ,
host.config.storageDevice.scsiLun.lunType : 1 }
);