In a previous post I showed how to get around a bug in Prototype that was ignoring the "on" parameter of the observe_field function. The solution I originally posted no longer works with the latest version of Rails and Prototype. Although the documentation for observe_field received a facelift, somehow the Rails team still missed the fact that the :on function doesn't work with the current version of Prototype. Or maybe its the Prototype guys we should be after here. :-)

The new fix requires a change to both prototype.js and a small Rails core update.

I will start with the update to prototype.js. Search for Abstract.EventObserver (around line 3632 in my version). That should now read

Abstract.EventObserver = Class.create({
  initialize: function(element, callback, trigger) {
    this.element  = $(element);
    this.callback = callback;
    this.trigger = trigger;

    this.lastValue = this.getValue();
    if (this.element.tagName.toLowerCase() == 'form')
      this.registerFormCallbacks();
    else
      this.registerCallback(this.element, this.trigger);
  },

  onElementEvent: function() {
    var value = this.getValue();
    if (this.lastValue != value) {
      this.callback(this.element, value);
      this.lastValue = value;
    }
  },

  registerFormCallbacks: function() {