Fragment caching is used for caching various blocks within views without caching the entire action as a whole. This is useful when certain elements of an action change frequently or depend on complicated state while other parts rarely change or can be shared amongst multiple parties. The caching is done using the cache helper available in the Action View. A template with fragment caching might look like:

<b>Hello <%= @name %></b>

<% cache do %>
  All the topics in the system:
  <%= render :partial => "topic", :collection => Topic.all %>
<% end %>

This cache will bind the name of the action that called it, so if this code was part of the view for the topics/list action, you would be able to invalidate it using:

expire_fragment(:controller => "topics", :action => "list")

This default behavior is limited if you need to cache multiple fragments per action or if the action itself is cached using caches_action. To remedy this, there is an option to qualify the name of the cached fragment by using the :action_suffix option:

<% cache(:action => "list", :action_suffix => "all_topics") do %>

That would result in a name such as /topics/list/all_topics, avoiding conflicts with the action cache and with any fragments that use a different suffix. Note that the URL doesn’t have to really exist or be callable

  • the url_for system is just used to generate unique cache names

that we can refer to when we need to expire the cache.

The expiration call for this example is:

expire_fragment(:controller => "topics", 
                :action => "list", 
                :action_suffix => "all_topics")
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May 31, 2010
4 thanks

Naming fragment cache

One of the common ways of using fragment caching is to cache content that’s shared across the site (eg. left navigation, menus, widgets etc.) that looks and works the same regardless of the name of the action or controller calling it. In such cases it’s very easy to just use named fragment caching eg.:

<% cache('left_nav') do -%>
  <%= display_left_nav -%>
<% end -%>
August 24, 2013
1 thank

expires_in option

@concept47 do you really need to check the fragment in the controller?

ActiveRecord will execute the query when its used

cars = Car.where(:colour => 'black') # No Query
cars.each {|c| puts c.name } # Fires "select * from cars where ..."

“Lazy Loading” - http://m.onkey.org/active-record-query-interface

October 2, 2013
0 thanks

You have explain well

You have explain well about fragment caching as I was in need of this information. http://autobodycarparts.com

August 15, 2013 - (v3.2.1 - v3.2.13)
0 thanks

expires_in option

You can actually pass in an expires_in option that sets how long Rails should show the fragment before deleting it so as an example …

<% cache('homepage_sidebar', :expires_in => 10.minutes) do %>
<% end %>

This only used to work with memcached but it now works with other types of Rails stores, MemoryStore, FileStore (had to use a plugin to get this behavior before) etc etc

So in your controller. You’d just do …

@posts = Posts.all if fragment_exists?('homepage_sidebar') 

to avoid performing a pointless SQL query.