Backbone.ModelWithState

// Subclass the Backbone model to keep the model and its state (errors)
// sent back by the server. Creates a new errorModel property holding
// a Backbone model with errors, mapped to the model attributes.
Backbone.ModelWithState = Backbone.Model.extend({
  errorModel: undefined,
  constructor: function(attributes, options) {
    Backbone.Model.apply(this, arguments);
    options || (options = {});
    this.errorModel = options.errorModel || new Backbone.Model();
  },
  save: function(key, val, options) {
    if (key === undefined) key = null;
    if (key == null || typeof key === 'object') options = val;
    options || (options = {});

    this.errorModel.clear();

    var error = options.error;
    options.error = function(model, resp, options) {
      if (resp && resp.responseJSON && _.isObject(resp.responseJSON))
        model.errorModel.set(resp.responseJSON);
      if (error) error(model, resp, options);
    };

    if (key == null || typeof key === 'object')
      return Backbone.Model.prototype.save.call(this, key, options);
    else
      return Backbone.Model.prototype.save.call(this, key, val, options);
  }
});