[{"virtualId":"BC01-BOOKNAME01","books":["BID01","BID02"]},{"virtualId":"BC02-BOOKNAME01","books":["BID03","BID04"]},{"virtualId":"~-BOOKNAME01","books":["BID05","BID06"]},{"virtualId":"BC02-~","books":["BID07","BID08"]},{"virtualId":"~-~","books":["BID09","BID10"]}]

db.collection.aggregate([{"$project":{books:1,virtualId:{"$function":{"body":"function(a){var t = '';for(i=0;i<a.length;i++){t=a.charCodeAt(i)+t;};return t;}","args":["$virtualId"],"lang":"js"}}}}])