_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 @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'

new.html.haml

%h1 New topic

= render 'form'

post_form.html.haml

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

rendered_html.html

    <h1>
      Edit topic
      mmmmmmmm
    </h1>
    <form accept-charset="UTF-8" action="/forums/ruby/topics/mmmmmmmm-932157ee-7b13-4af9-97af-7d99d668f73a" class="edit_topic" id="edit_topic_64" method="post"><div style="display:none"><input name="utf8" type="hidden" value="&#x2713;" /><input name="_method" type="hidden" value="patch" /><input name="authenticity_token" type="hidden" value="UQL4ExZ0s+J/pZvwhXpJGK6ZxcCVuTVDsfbY+OCEkrY=" /></div>  <label for="topic_name">Name</label>
      <div class='field'>
        <input id="topic_name" name="topic[name]" type="text" value="mmmmmmmm" />
      </div>
      <label for="topic_post_bodytext">Bodytext</label>
      <div class='field'>
        <textarea id="topic_post_bodytext" name="topic[post][bodytext]">
mmmmmmmmmmmmmmmmmmmmmmmmmmmmm</textarea>
      </div>  
      <div class='actions'>
        <input name="commit" type="submit" value="Update Topic" />
      </div>
    </form>

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
    @topic.posts.build
    @post = Post.find_by topic_id: @topic
    logger.debug(@post)
  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
    @topic = set_topic
    respond_to do |format|
      if @topic.update(topic_params)
        @topic.posts.build
        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