Call schema method without create a new Model object with Nodejs/Mongoose

My problem is that I want to update a user password that's already stored on MongoDB, and I can't call a method for User. Let me show the code:

User schema:

userSchema = new Schema({ ... });

userSchema.methods.setPassword = function (passwordPlainText) {
    this.passwordHash = createHash(passwordPlainText, this.salt);
};

module.exports = mongoose.model('User', userSchema);

And it works fine if I made this:

user = new User();
user.setPassword('foobar');

But if I want to do something like this:

User.findOne({email: req.param('email')}, function (err, user) {
    user.setPassword('foobar');
});

It outputs:

TypeError: Object #<Object> has no method 'setPassword'

Can someone please help me to find a way to call these schema methods after retrieving the user from database?

Additional info:

• node v0.8.4
• express v3.0.4
• mongoose module (http://mongoosejs.com/)