app.js

var connect = require('connect'),
  mongodb = require('mongodb');

var showDBInfo = function(req,res) { 
  res.end("You are connected to the database");
};

var noDatabaseError = function(res) {
  res.statusCode = 500;
  res.end("Unable to connect to database");
}

var vcap_service_connect = function(service_type,req,res,next) {
  var s = JSON.parse(process.env.VCAP_SERVICES)[service_type];
  if ( s == null || s.length < 1 ) {
    noDatabaseError(res);
  } else {
    s = s[0]["credentials"];
    var server = new mongodb.Server( s["host"], s["port"]);
    new mongodb.Db( s["db"], server, {} ).open( function(err,c) {
      c.authenticate( s["username"], s["password"], function(err,client) { 
        if ( err ) {
          noDatabaseError(res);
        } else {
          req.connection = connection = client;
          next();
        }
      });
    });
  }
}

var localhost_connect = function(name,req,res,next) {
  var server = new mongodb.Server("127.0.0.1",27017,{});
  new mongodb.Db( name, server, {}).open( function(err,client) { 
    if ( err ) {
        res.statusCode = 500;
        res.end("Unable to connect to database");
    } else {
        req.connection = connection = client;
        next();
    }
  });
}

var dbConnection = function(name) {
  var connection = null;
  return function(req,res,next) {
    if ( connection ) {
      req.connection = connection;
      next();
    } else {
      if ( process.env.VCAP_SERVICES ) {
        vcap_service_connect("mongodb-1.8",req,res,next);
      } else {
        localhost_connect(name,req,res,next);
      }
    }
  }
}

var port = process.env.VCAP_APP_PORT || process.env.PORT || 3000;
connect.createServer( dbConnection("middleware-test"), showDBInfo).listen(port);

package.json

{
 "name": "db-middleware",
 "version": "0.0.1",
 "dependencies":{
   "connect":"1.8.3",
   "mongodb":"0.9.7-2-2"
 }
}