title: real time copying mongoDB instance collection into new database
body: <p>I want to copy the database's specified collection into new database.
  I searched and found there is something Trigger technique which will update copying database whenever any modification happened in original database but it cost must so I want any other alternative solution.
  I also want rules for copying something like I only want few fields of particular collection however its not much important but main task is copying original database collection into new database in real time.
  We can say something like backup</p>