form.erb

<%= form_for @topic do |f| %>
  <%= debug f.object.error_messages %>
  <p>
    <%= f.label :name %><br />
    <%= f.text_field :name %>
  </p>

  <%= fields_for @post do |ff| %>
    <p>
      <%= ff.label :content %><br />
      <%= ff.text_field :name %>
    </p>
  <% end %>

  <p>
    <%= f.submit "Create" %>
  </p>
<% end %>

http://api.rubyonrails.org/classes/ActionView/Helpers/FormHelper.html#method-i-fields_for