var manager = function(db) {
        this.db = db;
    }
    manager.prototype = {
        doSth: function(cb) {
            this.db.findOne({}, function(err, doc) {
                // more logic here
                cb(doc);
            };    
        }
    }
     
    module.exports = manager;
           
    manager = require('manager');
    var db = require('mongo');
    db.connect(); //forget the details here
    var M = new manager(db);
     
    M.doSth();
           
    manager = require('manager');
    var M = new manager(app.get('db')); //ask the app for a resource or connect db again
    M.doSth();
           
    var manager = function() {
        this.db = null;
    }
    manager.prototype = {
        connect: function(db) {
            if (this.db == null)
                 this.db = db;
        }
        doSth: function(cb) {
            this.db.findOne({}, function(err, doc) {
                // more logic here
                cb(doc);
            };    
        }
    }
     
    module.exports = manager;
           
    M = require('manager');  //is already an instance
    var db = require('mongo').connect();
    M.connect(db)
    M.doSth();
           
    M = require('manager');
    M.doSth(); //will work since manager has been connected by app.js