Removendo um subdocumento do array no MongodDB

db.so.drop();
db.so.insert(
{
    "_id" : ObjectId("51ee899ec15d5aaff39d3353"),
    "first" : "first",
    "last" : "last",
    "email" : "email",
    "phone" : "555 555 5555",
    "userRole" : "su",
    "cards" : [
            {
                    "rfid_id" : ObjectId("51ee899ec15d5aaff39d3359")
            }
    ],
    "zones" : [
            {
                    "zone_id" : ObjectId("51ee899ec15d5aaff39d3357")
            }
    ]
});

db.so.update({}, {$pull:{ "cards": {"rfid_id": ObjectId("51ee899ec15d5aaff39d3359")}}}, false, false)
db.so.find().pretty();

{
    "_id" : ObjectId("51ee899ec15d5aaff39d3353"),
    "cards" : [ ],
    "email" : "email",
    "first" : "first",
    "last" : "last",
    "phone" : "555 555 5555",
    "userRole" : "su",
    "zones" : [
        {
            "zone_id" : ObjectId("51ee899ec15d5aaff39d3357")
        }
    ]
}