undefined method 'act_as_tree'

I have 2 models: topic model and post:

class Topic < ActiveRecord::Base
  has_many :posts
end

class Post < ActiveRecord::Base
  acts_as_tree :order=>"body"
  belongs_to :topic
end

and I have a view for creating new topic (and new post):

<h1>Add a new Topic</h1>

<% form_for :topic, @topic, :url => { :action => "create" } do |f| %>
  Title: <%= f.text_field :title %>

  <% fields_for :posts do |post| %>
    Body:<%= post.text_field :body %><br/>
  <% end %>

  <%= submit_tag 'Post Topic', :class => "submit" %>
<% end %>

I get the undefined method 'act_as_tree' error in the
TopicsController: (in the create method)
@post = Post.new(params[:post])

any ideas?

What version of rails are you running? In 2.0 acts_as_tree (along with a bunch of stuff) has been moved into separate plugins.

Fred

edge..
I guess I need to google for this plug-in.

thanks!