NilClass in Partial

I appreciate your help in advance; I feel like the issue is staring me
in the face!

I have two models: event and topic

/models/event.rb:
class Event < ActiveRecord::Base
  ..
  has_many :topics
  ..

/models/topic.rb:
class Topic < ActiveRecord::Base
  belongs_to :event
end

I'm successfully showing the dependent topics with the event:
/views/events/show.html.erb:

..
<table>
<%@event.topics.each do |topic| %>
<tr>
  <td> <%= topic.Name %> </td>
  <td> <%= topic.Description %></td>
</tr>
<% end %>
</table>
..

When I attempt to edit the record that I can successfully show, I get:

Showing /views/events/_topic.html.erb where line #2 raised:

undefined method `new_record?' for nil:NilClass
Extracted source (around line #2):

1: <div class="topic">
2: <% new_or_existing = topic.new_record? ? 'new': 'existing' %>
3: <% prefix = "event[#{new_or_existing}_topic_attributes][]"%>
4:
5: <% fields_for prefix, topic do |topic_form| -%>

The partial is rendered in edit.html.erb:

<h1>Editing tag</h1>

<%= render 'form' %>
      <div id="topicContentArea">
        <%= render 'topic', :collection => @event.topics %>
      </div>
<%= link_to 'Show', @event %> |
<%= link_to 'Back', events_path %>

Ideas are appreciated!
Thanks,
Karl

I have not used :collection to pass something like @event.topics
rather than a simple variable such as @topics, so wonder whether this
works as you expect. You could try
<%= render 'topic', :collection => @event.topics, :as => :topic %>
in order to force the variable name in the partial.

Colin