gistfile1.js

var mongoose = require('mongoose')
  , Schema =  mongoose.Schema;

function NotFound(msg){
  this.name = 'NotFound';
  Error.call(this, msg);
  Error.captureStackTrace(this, arguments.callee);
}

NotFound.prototype.__proto__ = Error.prototype;

mongoose.model('User', new Schema({
  "name": {type: String, required: true, index: true}
}));

mongoose.connect('mongodb://localhost/foo-db', function(err) {if (err) throw err;});

var app = require('express').createServer();

app.error(function(err, req, res, next){
    if (err instanceof NotFound) {
        res.send({code:404, message: err.message||'Not Found'}, 404);
    } else {
        res.send({code:500, message: err.messgae||'Internal error', stack:err.stack.split('\n')}, 500);
    }
});

var findUserByIdAndAttachToRequest = function(prop) {
  return function(req, res, next, id) {
    console.log('Search user #'+id+' and put in req.'+prop);
    mongoose.model('User').findById(id, function(err, user) {
      if (err)   return next(err);
      if (!user) return next(new NotFound('User not found'));
      req[prop] = user;
      next();
    });
  };
};
app.param('from_id', findUserByIdAndAttachToRequest('_fromUser'));
app.param('to_id',   findUserByIdAndAttachToRequest('_toUser'));
app.get('/users/:from_id/friend/:to_id', function(req, res, next) {
  res.send({formUser:req._fromUser.name, toUser:req._toUser.name});
});
app.listen(3000);