Mongo db 2 level array with indexes search

Question

I have this arbitrary structure of a collection. Need to find a match on itemId. Here's the structure:

{
    "_id" : "uHof564CDJcnvFHKz",
    "user_id" : "dukcCKjFcQe2v7bHD",
    "cart" : {
        "createdAt" : ISODate("2016-04-19T18:12:52.456Z"),
        "items" : [ 
            {
                "itemId" : "FfK49XAcX7YR",
                "itemPrice" : "10",
                "shortName" : "Item Description",
                "itemType" : "item1"
            },
             {
                "itemId" : "RgX6FfuK49XAcX",
                "itemPrice" : "20",
                "shortName" : "Item Description",
                "itemType" : "item2"
            }         
        ]
    }

So, I have an itemId and need to find a match in "cart -- items".

I tried $elemMatch, and multiple $elemMatch that did not work, returned null.