code: function (req, res, next, id) {
      var self = this;
  
      // check if a custom id is used, when not only process a valid object id
      if (this.mongoId && !ObjectID.isValid(id)) {
        res.badRequest();
        return next();
      }
  
      // attach the document as this.paramName to the request
      this.model.findOne({'_id': id}, function (err, doc) {
        if (err) {
          return next(err);
        }
  
        if (!doc) {
          res.notFound();
          return next('route');
        }
  
        req[self.paramName] = doc;
        return next();
      });
    }
docs: Register the default id parameter for route requests.
  Add a property to the current request which is the
  document returned by the controller Model for the param configures
  paramter name available in the processed request.
  @param {http.IncomingMessage} req - The request message object
  @param {http.ServerResponse} res - The outgoing response object
  @param next {function} - The next handler function to call when done
  @param id {String} - The id parameter parsed from the current request
  @returns {function} This function sets a status of 400 for malformed MongoDB
  id's and a status of 404 if no document has been found for the passed
  parameter value. Calls the passed next function when done.