In which controller should I place elements that persist throughout the application? - ruby-on-rails

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.