_form.html.haml

= form_for([@forum, @topic]) do |f|
  - if @topic.errors.any?
    %div{id: 'error_explanation'}
      %h2
        = pluralize(@topic.errors.count, "error") 
        prohibited this topic from being saved:
      - for message in @topic.errors.full_messages
        %li
          = message

  = f.label :name
  %div{class: 'field'}
    = f.text_field :name

  = f.fields_for :posts, post do |p|
    = render :partial => "posts/form", :locals => { :f => p }

  %div{class: 'actions'}
    = f.submit

- if @topic.new_record?
  asd

edit.html.haml

%h1
  Edit topic 
  = @topic.name
= render 'form', post: @post

log-file.log

Started PATCH "/forums/ruby/topics/mmmmmmmm-932157ee-7b13-4af9-97af-7d99d668f73a" for 127.0.0.1 at 2014-04-25 11:10:56 +0200
Processing by TopicsController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"UQL4ExZ0s+J/pZvwhXpJGK6ZxcCVuTVDsfbY+OCEkrY=", "topic"=>{"name"=>"Lorem Ipsum", "post"=>{"bodytext"=>"Lorem Ipsum is simply dummy text of the printing and typesetting industry. Lorem Ipsum has been the industry's standard dummy text ever since the 1500s, when an unknown printer took a galley of type and scrambled it to make a type specimen book. It has survived not only five centuries, but also the leap into electronic typesetting, remaining essentially unchanged. It was popularised in the 1960s with the release of Letraset sheets containing Lorem Ipsum passages, and more recently with desktop publishing software like Aldus PageMaker including versions of Lorem Ipsum."}}, "commit"=>"Update Topic", "forum_id"=>"ruby", "id"=>"mmmmmmmm-932157ee-7b13-4af9-97af-7d99d668f73a"}

new.html.haml

%h1 New topic

= render 'form', post: @topic.posts.first

post_form.html.haml

= f.label :bodytext
%div{class: 'field'}
  = f.text_area :bodytext

topics_controller.rb

class TopicsController < ApplicationController
  before_filter :find_forum
  before_filter :authorize, only: [:new, :edit, :update]
  before_action :set_topic, only: [:show, :edit, :update]

  def index 
    redirect_to forum_path(@forum)
  end

  def show
    @topic = Topic.friendly.find(set_topic)
    @posts = @topic.posts
  end

  def new
    @topic = Topic.new
    @topic.posts.build
  end

  def edit
    @post = @topic.posts.first
  end

  def create
    @topic = @forum.topics.build(topic_params)
    @topic.author_id = current_user.id
    logger.debug(topic_params)

    respond_to do |format|
      if @topic.save
        format.html { redirect_to forum_topic_path(@forum, @topic), notice: 'Topic was successfully created.' }
      else
        format.html { render :new }
      end
    end
  end

  def update
    respond_to do |format|
      if @topic.update(topic_params)
        format.html { redirect_to forum_topic_path(@forum, @topic), notice: 'Topic was successfully updated.' }
      else
        format.html { render :edit }
      end
    end
  end

  private
    def find_forum
      @forum = Forum.find(params[:forum_id])
    end

    def set_topic
      @topic = Topic.find(params[:id])
    end

    # Allow only the white list
    def topic_params
      params.require(:topic).permit(:name, author_id: current_user.id , posts_attributes: [ :bodytext ])
    end
end