It's a code sample bonus! If you implement the data model from the previous post you'll quickly notice that plain old Rails "form_for(@widgets)" in  widget/edit won't help you update @widget.widget_instances, and creating a controller/view for widget_instances is the last thing you want to do. If you're using map.resources :widgets in routes you'll have problems with the form_tag approach however - Rails gets confused and won't understand that /widgets/1 means show in this instance. This however, will work:

<% form_tag("/widgets/update/".concat(@widget.id.to_s), :method => :put) do%>

This form signature is useful anytime you want to stay within map.resources but update tables outside the model in question with your update.