Question

Everything after @posts should persist throughout the application (Like the left sidebar you find on Facebook).

  def index
    @title = "Posts"
    default_order = "content_changed_at DESC"
    params[:order_by] ||= default_order 
    @posts = current_user.subscribed_posts.paginate(:page => params[:page],
                                                    :per_page => 5,
                                                    :order => params[:order_by])
    @subscribed_tags = current_user.subscribed_tags
    @recent_posts = current_user.posts.limit(5).order("created_at DESC")
    @tags= Tag.limit(20).order("ID asc")
    @user = current_user
  end

views/layout/_sidebar.html.erb:

    <div class="user-profile">
      <% avatar = image_tag(current_user.avatar.url(:thumb), :class => "authenticated-avatar") %>
      <%= link_to avatar, "/users/#{current_user.id}" %>
      <%= link_to "#{current_user.username}", "/users/#{current_user.id}", :class => "authenticated-username" %>
    </div>

    <%= form_for(@user, :remote => true) do |f| %>
      <div class="field">
        <%= f.label :subscribed_tag_names %><br />  
        <%= f.autocomplete_field :subscribed_tag_names, autocomplete_tag_name_posts_path, :"data-delimiter" => ' ', :class => "autocomplete_field" %>
      </div>
      <div class="actions">
        <%= f.submit :class => "user_subscribed_tag_names_submit" %>
      </div>
    <% end %>

    <div class="user-subscribed_tags">
      <% @subscribed_tags.each do |subscribed_tag| %>
        <%= link_to "#{subscribed_tag.name}(#{subscribed_tag.posts.count})", unsubscribe_tags_path(:unsubscribed_tag_name => subscribed_tag.name) %>
      <% end %>
    </div>

    <div class="user-recent-posts">
      <h4>Recent Posts</h4>
      <ul>
        <% @recent_posts.each do |recent_post| %>
          <li><%= link_to recent_post.title, recent_post %></li>
        <% end %>
      </ul>
    </div>

    <div class="top-tags">
      <% @tags.each do |tag| %>
        <span class="tag-name"><%= tag.name %></span>
        <span class="tag-count"><%= tag.posts.count %></span>
      <% end %>
    </div>
  <% end %>

Where should I place the code in the controller if I want them to persist throughout the application? (I would like to see some example code if possible).

Answer 1

If it persists throughout the application, i.e. for every actions of all controllers, you might place it in a before_filter in the application controller.

Add a before_filter to the application controller as :

before_filter :find_recent_posts_and_tags

And define it (as private) :

private
def find_recent_posts_and_tags
  # define your instance variables
end

More about filters here and here.