I have a mongoose model that has an association with a user model e.g.
var exampleSchema = mongoose.Schema({
name: String,
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.