use CouchDB _id

couchmodel.js

var oldSet = Backbone.Model.prototype.set;
  
_.extend(Backbone.Model.prototype, {
  set: function(attrs, options) {                                     
    if ('_id' in attrs) this.id = attrs._id;
    oldSet.apply(this, [attrs, options]);                             
    return this;
  }   
});