mongoClient = new MongoClient(new MongoClientURI(uri));
    // Set the desired Database
    DB mDB = mongoClient.getDB(db);
    DBCollection collection = mDB.getCollection("objectstore");
    BasicDBObject mQuery = new BasicDBObject("data.vars",new     BasicDBObject("$exists",true));
// create our pipeline operations, first with the $match
            DBObject match = new BasicDBObject("$match", new BasicDBObject("data.vars", new BasicDBObject("$exists",true)));

            // build the $projection operation
            DBObject pFields = new BasicDBObject("_id", 1);
            pFields.put("unitValueMid","$data.vars.unitValueMid");
            DBObject project = new BasicDBObject("$project", pFields );

            // run aggregation
            List<DBObject> pipeline = Arrays.asList(match, project);
            AggregationOutput output = collection.aggregate(pipeline);