[{"_id":"1923","myItem":{"id":"98syf87erfw8n","foo":{"objectid":"39","stuff":"65"},"yearpublished":"1979"}},{"_id":"1453","myItem":{"id":88888888888,"foo":{"objectid":"394","stuff":"55"},"yearpublished":"1453","author":"Ravi Kiran"}}]

db.collection.aggregate({"$replaceRoot":{"newRoot":"$myItem"}})