gistfile1.rb

<%= form_for(@comment, @code) do |f| %>
  <%= f.hidden_field :commentable, :value => commentable.class.name %>
  <%= f.hidden_field :commentable_id, :value => commentable.id %>
  <%= f.label :content %><br />
  <%= f.text_area :content %><br />
  <%= f.fields_for @code do |c| %> 
     <div class="field">
      <%= c.select "file_type", SourceCode::FILE_TYPES %>
      <%= c.label :source %><br />
      <%= c.text_area :source %>
      </div>
    <% end %>
    <%= f.submit %>
<% end %>

gistfile2.txt

   <%= render :partial => 'comments/form', :locals => {:commentable => @question} %>

gistfile3.txt

# part of the controller to determine class


      c = params[:comment]
      @commentable = nil
      if c[:commentable]== "Question"
         @commentable = Question.find(c[:commentable_id]) 
      elsif c[:commentable] == "Snippet"
         @commentable = Snippet.find(c[:commentable_id]) 
      end