<%= edit_form_for(forum_post) do |f| %>
  <% if forum_post.topic_id.present? %>
    <%= f.input :topic_id, :as => :hidden %>
  <% else %>
    <%= f.input :topic_id, :label => "Topic ID" %>
  <% end %>
  <%= f.input :body, as: :dtext %>

  <%= f.button :submit, "Submit" %>
  <%= dtext_preview_button "forum_post_body" %>
<% end %>
