memcached set sub-object in node.js

      // memcached set up
      var nMemcached = require('memcached');
      var memcached = new nMemcached(['192.168.200.3:11211']);

      // original data structure
      var room_tokens = {};

      // some initial game value
      var appinfo = 'poker'
        , token = 'ABCDEFGHI'
        , slots = 4
        , joining_time = 30000
        , display = 'display_' + token;

      // register to room_tokens to find it later & init room properties
      // INTENT TO DO
      room_tokens[token] = {
        slot_available: slots,
        slot_left: slots,
        joining_time_left: joining_time,
        user_list: {},
        game_state: 0//0: selection; 1: game started
      };
      // MEMCACHED APPROACH
      memcached.set(
        appinfo + '_' + token, {
          slot_available: slots,
          slot_left: slots,
          joining_time_left: joining_time,
          user_list: {},
          game_state: 0//0: selection; 1: game started
        },
        10000,
        function(err, result){
          if(err) {
            console.error(err);
          }
          console.dir(result);
        }
      );

      // register display to the room for later fast duplicate name indexing
      // INTENT TO DO
      room_tokens[token]['user_list'][user_name] = display;
      // MEMCACHED APPROACH. This will result the object got wipe out to only user_list sub-object
      memcached.set(
        appinfo + '_' + token, {
          user_list: { ''+user_name: display }
        },
        10000,
        function(err, result){
          if(err) {
            console.error(err);
          }
          console.dir(result);
        }
      );

      // note that I could memcached.get() then memcached.set() to do the updating,
      // but have to do it every time