Backbone.js sents POST instead of PUT (mongoose set _id to id)
From the fine manual http://backbonejs.org/#Model-idAttribute
idAttribute model.idAttribute
A model's unique identifier is stored under the id attribute. If you're directly communicating with a backend (CouchDB, MongoDB) that uses a different unique key, you may set a Model's idAttribute to transparently map from that key to id.
Id attribute can be changed per model like
var Meal = Backbone.Model.extend({ idAttribute: "_id"});To make it default for all models (eg., if we are using mongodb), override the default setting by placing this LOC in your app js file that runs after backbone.js
//override the id attribute for modelBackbone.Model.prototype.idAttribute = '_id';