gistfile1.rb

# i18n
en:
  activerecord:
    models:
      topic: Topic
    attributes:
      topic:
        posts_body: Body

# topic
class Topic < ActiveRecord::Base
  attr_accessor :body
  has_many :posts,  :autosave => true 
  validates_presence_of :title
end

# post
class Post < ActiveRecord::Base
  belongs_to :topic
  validates_presence_of :body
end

# topic_controller/create action
def create
  @topic = Topic.new(params[:topic])
  @topic.posts << Post.new(:body => params[:topic][:body])
  @topic.save ? redirect_to(@topic) : render :action => :new
end

# new.html.erb
<h1>Create new topic</h1>
<%= error_messages_for :topic %>
<% form_for @topic do |f| -%>
  <p>
    <label>title</label><br/>
    <%= f.text_field :title %>
  </p>
  <p>
    <label>body</label><br/>
    <%= f.text_area :body %>
  </p>
  <p>
    <%= f.submit 'create' %>
  </p>
<% end -%>