Rails 3 upgrade part 4: Prototype helpers and Javascript
Rails 3 is embracing the unobtrusive Javascript (or UJS) mantra which is good because it is the right way; at the same time, it is bad because many applications will break when they upgrade to Rails 3. On the other hand, who’s expecting a smooth upgrade anyway 
In my test application, I used jrails because I am more interested in jQuery than Prototype. But since jrails doesn’t work with Rails 3, I removed it.
When jrails was removed, I received this error:
undefined method `observe_field' for #<#<Class:0xb6867e58>:0xb6865b6c>
Install Prototype helper plugin
‘observe_field’ is a Prototype helper and Rails 3 removed the the link between its Javascript helpers and Prototype. The goal in Rails 3 is for developers to use their preferred Javascript library. Also note that remote_#{method} helpers have been removed from Rails and moved to Prototype Legacy Helper plugin . To install this plugin, just do:
rails plugin install git://github.com/rails/prototype_legacy_helper
Remove jQuery
Once the prototype_legacy_helper is installed, the missing method is gone but observe_field is not triggering. Removing jQuery fixes this problem.
Now what if you want to use jQuery instead of Prototype? It depends how dependent your application is to Prototype. I have not found a jQuery equivalent for Prototype helper plugin yet so that would be an issue like in my case. Based on this jQuery and Rails 3 tutorial, using the jQuery UJS driver looks very easy.
