Method to only send some Backbone Model attributes to server.

payload.js

var UserModel = Backbone.Model.extend({

  defaults: {
    username: "",
    password: {
      old: "",
      new: "",
      confirm: ""
    }
  },

  validate: function(attrs) {
    if (attrs.password.new !== attrs.password.confirm) {
      return "Passwords must match";
    }
  },

  payload: function() {
    var payload = this.toJSON();
    delete payload.password.confirm;
    return payload;
  },

  sync: function(method, model, options) {
    if (method == 'update' || method == 'create') {
      options.attrs = model.payload();
      return Backbone.sync.call(model, method, model, options);
    } else {
      return Backbone.sync.call(this, method, this, options);
    }
  }

});