class TopicContent < ActiveRecord::Base
 unloadable
 belongs_to :topic
 validates_presence_of :text
end

<%= f.fields_for :topic_content do |tf| %>
  <%= tf.text_area  :text , :id => 'topic_text',  :cols => 100 , :rows => 15, :class => 'topic_text' %></p>

<% @topic.errors.full_messages.each do |msg| %>
   <li><%= msg %></li>
  <% end %>