module ActionView::Helpers::CacheHelper

def cache_if(condition, name = {}, options = {}, &block)

<% end %>
<%= render project.topics %>
All the topics on this project
<% cache_if admin?, project do %>

Cache fragments of a view if +condition+ is true 
def cache_if(condition, name = {}, options = {}, &block)
  if condition
    cache(name, options, &block)
  else
    yield
  end
  nil
end