# STI Models
class Question < AR::Base; end        # should never be instantiated
class YesNoQuestion < Question; end   # should
class TextQuestion < Question; end    # should
# etc...

# Form.rb (model)
accepts_nested_attributes_for :questions # except I don't want to instantiate Question!

# forms/edit.html.erb (view)
...normal form_for |f|...
<% f.fields_for :questions do |question_form| %>
  <%= question_form.select :type, { "Yes/No" => "YesNoQuestion", "Text" => "TextQuestion" } %>
<% end %>