Passing model parameters into a mongoose model

_ = require("underscore")

var model = new Example(_.extend({ userId: req.user._id }, req.body))

or if you want to copy userId into req.body:

var model = new Example(_.extend(req.body, { userId: req.user._id }))

If I understood you correctly, you'll be good trying the following:

// We "copy" the request body to not modify the original one
var example = Object.create( req.body );

// Now we add to this the user id
example.userId = req.user._id;

// And finally...
var model = new Example( example );

Also, do not forget to add in your Schema options { strict: true }, otherwise you may be saving unwanted/attackers data.