NedB on heroku dont keep db file

i used nedb in my nodejs app , first of my code is :

var Datastore = require('nedb');
var db = new Datastore({ filename: 'users.db', autoload: true });
db.persistence.setAutocompactionInterval(5);

everything is ok with local host , and database will save every thick in my db file . (Database saved after server restarting) but in heroku server , database is empty !!! i think database saved as in-memory ! (not in my db file !!) Why ?! i want keep my database in my db file in Heroku server !

via Mahdi Safarloo