node.js email verification token

I'm trying to set up a verification step in Mongoose, Express Node application based on this blog post ... http://danielstudds.com/adding-verify-urls-to-an-express-js-app-to-confirm-user-emails-secure-spa-part-6/ that post is over a year old so it kind of surprises me that its the first google result for 'node email verification'. I'm very new to node so I'm reliant on examples. Based on that post I did not see a download so I pieced it together to suit my scenario and here is what my code looks like.

Verification Model

var mongoose = require('mongoose'),
    uuid = require('node-uuid'),
    User = require('mongoose').model('User');

var verificationTokenSchema = new mongoose.Schema({
   _userid : { type: mongoose.Schema.Types.ObjectId, ref: 'User' },
   token: {type: String, required: true},
   createdAt: {type: Date, required: true, default: Date.now, expires: '4h'}
});

verificationTokenSchema.methods = {
   createVerificationToken : function (done) {

      var verificationToken = this;
      var token = uuid.v4();
      verificationToken.set('token', token);
      verificationToken.save( function (err) {
         if (err) return done(err);
            return done(null, token);
      });
   }
};

exports.verifyUser = function(token, done) {
   verificationTokenModel.findOne({token: token}, function (err, doc){
      if (err) return done(err);
      User.findOne({_id: doc._userId}, function (err, user) {
         if (err) return done(err);
         user["verified"] = true;
         user.save(function(err) {
            done(err);
         });
      });
   });
};


var verificationTokenModel = mongoose.model('VerificationToken', verificationTokenSchema);
exports.verificationTokenModel = verificationTokenModel;

Then in my User model I call create like so..

User Model

exports.createUser = function(req, res, next) {
   // Do all the stuff that creates the user save it and get the id back
   var verificationToken = new verificationTokenModel({_userId: user._id});
   verificationToken.createVerificationToken(function (err, token) {
       if (err){
           err = new Error("Couldn't create verification token");
           res.status(400);
           return res.send({reason:err.toString()});
        }

        // Do stuff with the token here and email

This works 'partially' in my db 'verificationtokens' collection the objects don't contain a _userid they contain the _userid (user._id) stored in _id

My first issue is I don't really understand how this works when there isn't a 'constructor'

var verificationToken = new verificationTokenModel({_userId: user._id});

and how do I get this to store the user._id as _userid in the verification collection