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")
Methods
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Instance Public methods
expire_fragment(key, options = nil)

Removes fragments from the cache.

key can take one of three forms:

  • String - This would normally take the form of a path, like pages/45/notes.

  • Hash - Treated as an implicit call to url_for, like {:controller => "pages", :action => "notes", :id => 45}

  • Regexp - Will remove any fragment that matches, so %r{pages/\d*/notes} might remove all notes. Make sure you don't use anchors in the regex (^ or $) because the actual filename matched looks like ./cache/filename/path.cache. Note: Regexp expiration is only supported on caches that can iterate over all keys (unlike memcached).

options is passed through to the cache store's delete method (or delete_matched, for Regexp keys.)

Source: show | on GitHub 

# File actionpack/lib/action_controller/caching/fragments.rb, line 109
def expire_fragment(key, options = nil)
  return unless cache_configured?
  key = fragment_cache_key(key) unless key.is_a?(Regexp)

  instrument_fragment_cache :expire_fragment, key do
    if key.is_a?(Regexp)
      cache_store.delete_matched(key, options)
    else
      cache_store.delete(key, options)
    end
  end
end
fragment_cache_key(key)

Given a key (as described in expire_fragment), returns a key suitable for use in reading, writing, or expiring a cached fragment. If the key is a hash, the generated key is the return value of url_for on that hash (without the protocol). All keys are prefixed with views/ and uses ActiveSupport::Cache.expand_cache_key for the expansion.

Source: show | on GitHub 

# File actionpack/lib/action_controller/caching/fragments.rb, line 51
def fragment_cache_key(key)
  ActiveSupport::Cache.expand_cache_key(key.is_a?(Hash) ? url_for(key).split("://").last : key, :views)
end
fragment_exist?(key, options = nil)

Check if a cached fragment from the location signified by key exists (see expire_fragment for acceptable formats)

Source: show | on GitHub 

# File actionpack/lib/action_controller/caching/fragments.rb, line 82
def fragment_exist?(key, options = nil)
  return unless cache_configured?
  key = fragment_cache_key(key)

  instrument_fragment_cache :exist_fragment?, key do
    cache_store.exist?(key, options)
  end
end
instrument_fragment_cache(name, key)

Source: show | on GitHub 

# File actionpack/lib/action_controller/caching/fragments.rb, line 122
def instrument_fragment_cache(name, key)
  ActiveSupport::Notifications.instrument("#{name}.action_controller", :key => key){ yield }
end
read_fragment(key, options = nil)

Reads a cached fragment from the location signified by key (see expire_fragment for acceptable formats).

Source: show | on GitHub 

# File actionpack/lib/action_controller/caching/fragments.rb, line 70
def read_fragment(key, options = nil)
  return unless cache_configured?

  key = fragment_cache_key(key)
  instrument_fragment_cache :read_fragment, key do
    result = cache_store.read(key, options)
    result.respond_to?(:html_safe) ? result.html_safe : result
  end
end
write_fragment(key, content, options = nil)

Writes content to the location signified by key (see expire_fragment for acceptable formats).

Source: show | on GitHub 

# File actionpack/lib/action_controller/caching/fragments.rb, line 57
def write_fragment(key, content, options = nil)
  return content unless cache_configured?

  key = fragment_cache_key(key)
  instrument_fragment_cache :write_fragment, key do
    content = content.to_str
    cache_store.write(key, content, options)
  end
  content
end