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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. See ActionView::Helpers::CacheHelper for more information.

While it's strongly recommended that you use key-based cache expiration (see links in CacheHelper for more information), it is also possible to manually expire caches. For example:

expire_fragment('name_of_cache')
Namespace
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).

# File actionpack/lib/action_controller/caching/fragments.rb, line 124
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. All keys begin with views/, followed by any controller-wide key prefix values, ending with the specified key value. The key is expanded using ActiveSupport::Cache.expand_cache_key.

# File actionpack/lib/action_controller/caching/fragments.rb, line 64
def fragment_cache_key(key)
  head = self.class.fragment_cache_keys.map { |k| instance_exec(&k) }
  tail = key.is_a?(Hash) ? url_for(key).split("://").last : key
  ActiveSupport::Cache.expand_cache_key([*head, *tail], :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).

# File actionpack/lib/action_controller/caching/fragments.rb, line 97
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
read_fragment(key, options = nil)

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

# File actionpack/lib/action_controller/caching/fragments.rb, line 85
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).

# File actionpack/lib/action_controller/caching/fragments.rb, line 72
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