docs: function (id, cb) {
      var self = this;
      var idObj = {};
      idObj[this.model.prototype.idAttribute] = id;
      var model = new this.model(idObj, {collection: this});
      model.fetch({
        callback: function (err) {
          if(err){
            delete model.collection;
            if (cb){
              cb(Error('not found'));
            }
            return;
          }
          self.add(model);
          if (cb){
            return cb(null, model);
          }
        }
      });
    }