_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 do |t|
    = t.label :bodytext
    %div{class: 'field'}
      = t.text_area :bodytext

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

generated_html.html

    <h1>
      Edit topic
      123123111555
    </h1>
    <form accept-charset="UTF-8" action="/forums/ruby/topics/123123111" class="edit_topic" id="edit_topic_59" 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="123123111555" />
      </div>
      <label for="topic_posts_attributes_0_bodytext">Bodytext</label>
      <div class='field'>
        <textarea id="topic_posts_attributes_0_bodytext" name="topic[posts_attributes][0][bodytext]">
mmmmmmmmmmmmmmmaaaaaaaaaaaa</textarea>
      </div>
      <input id="topic_posts_attributes_0_id" name="topic[posts_attributes][0][id]" type="hidden" value="58" /><label for="topic_posts_attributes_1_bodytext">Bodytext</label>
      <div class='field'>
        <textarea id="topic_posts_attributes_1_bodytext" name="topic[posts_attributes][1][bodytext]">
mmmmmmmmmmmmmmmaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</textarea>
      </div>
      <input id="topic_posts_attributes_1_id" name="topic[posts_attributes][1][id]" type="hidden" value="59" /><label for="topic_posts_attributes_2_bodytext">Bodytext</label>
      <div class='field'>
        <textarea id="topic_posts_attributes_2_bodytext" name="topic[posts_attributes][2][bodytext]">
mmmmmmmmmmmmmmmaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa12312123123</textarea>
      </div>
      <input id="topic_posts_attributes_2_id" name="topic[posts_attributes][2][id]" type="hidden" value="60" /><label for="topic_posts_attributes_3_bodytext">Bodytext</label>
      <div class='field'>
        <textarea id="topic_posts_attributes_3_bodytext" name="topic[posts_attributes][3][bodytext]">
mmmmmmmmmmmmmmmaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa</textarea>
      </div>
      <input id="topic_posts_attributes_3_id" name="topic[posts_attributes][3][id]" type="hidden" value="61" />  
      <div class='actions'>
        <input name="commit" type="submit" value="Update Topic" />
      </div>
    </form>

logfile.log

Started PATCH "/forums/ruby/topics/123123111" for 127.0.0.1 at 2014-04-25 10:19:50 +0200
Processing by TopicsController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"UQL4ExZ0s+J/pZvwhXpJGK6ZxcCVuTVDsfbY+OCEkrY=", "topic"=>{"name"=>"123123111555", "posts_attributes"=>{"0"=>{"bodytext"=>"mmmmmmmmmmmmmmmaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa12312123123", "id"=>"58"}, "1"=>{"bodytext"=>"mmmmmmmmmmmmmmmaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", "id"=>"59"}}}, "commit"=>"Update Topic", "forum_id"=>"ruby", "id"=>"123123111"}

routes.rb

Rails.application.routes.draw do
  root to: 'categories#index'

  #get 'forum/:id', to: 'forums#show', as: 'forums'

  #get 'forum/:forum_id/:id', to: 'topics#show', as: 'topics'

  resources :forums, only: [:index, :show] do
    resources :topics do
      resources :posts, only: [:index]
    end
  end
end

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 = set_topic
  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