docs: function (id, options, cb) {
      if (arguments.length !== 3) {
        cb = options;
        options = {};
      }
      var self = this;
      var model = this.get(id);
      if (model){
        return cb(null, model);
      }
      function done() {
        var model = self.get(id);
        if (model) {
          if (cb){
            cb(null, model);
          }
        } else {
          cb(new Error('not found'));
        }
      }
      if (options.all) {
        this.fetch({
          callback: done
        });
      } else {
        this.fetchById(id, cb);
      }
    }