def update
    @post = Post.find(params[:id])
    @categories = Category.all
    @post.update_attributes(params[:post])
    respond_with(@post)
  end

---------------------------------

= form_for(@post) do |f|
 - if @post.errors.any?
    div#error_explanation
     h2
      = pluralize(@post.errors.count, "error")
      | prohibited this post from being saved:
     ul
      - @post.errors.full_messages.each do |msg|
       li = msg
 div.field
  = f.label 'Заголовок:'
  br
  = f.text_field :title
 div.field
  = f.label 'Категории:'
  br
  = f.select(:category_ids, options_from_collection_for_select(@categories = Category.all, :id, :title, :selected => @post.category_ids), {}, {:multiple => true, :size => 7})
 div.field
  = f.label 'Текст:'
  br
  = f.text_area :content
 div.field
  = f.label :tags
  br
  = f.text_field :tag_list
 div.actions
  = f.submit 'Опубликовать'