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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
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Instance Public methods
combined_fragment_cache_key(key)

Given a key (as described in expire_fragment), returns a key array suitable for use in reading, writing, or expiring a cached fragment. All keys begin with :views, followed by ENV or ENV if set, followed by any controller-wide key prefix values, ending with the specified key value.

# File actionpack/lib/abstract_controller/caching/fragments.rb, line 88
def combined_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
  [ :views, (ENV["RAILS_CACHE_ID"] || ENV["RAILS_APP_VERSION"]), *head, *tail ].compact
end
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/abstract_controller/caching/fragments.rb, line 148
  def expire_fragment(key, options = nil)
    return unless cache_configured?
    key = combined_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

  def instrument_fragment_cache(name, key) # :nodoc:
    ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}", instrument_payload(key)) { yield }
  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/abstract_controller/caching/fragments.rb, line 69
      def fragment_cache_key(key)
        ActiveSupport::Deprecation.warn(<<-MSG.squish)
          Calling fragment_cache_key directly is deprecated and will be removed in Rails 6.0.
          All fragment accessors now use the combined_fragment_cache_key method that retains the key as an array,
          such that the caching stores can interrogate the parts for cache versions used in
          recyclable cache keys.
        MSG

        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/abstract_controller/caching/fragments.rb, line 121
    def fragment_exist?(key, options = nil)
      return unless cache_configured?
      key = combined_fragment_cache_key(key)

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

    # Removes fragments from the cache.
    #
    # +key+ can take one of three forms:
    #
    # * String - This would normally take the form of a path, like
    #   <tt>pages/45/notes</tt>.
    # * Hash - Treated as an implicit call to +url_for+, like
    #   <tt>{ controller: 'pages', action: 'notes', id: 45}</tt>
    # * Regexp - Will remove any fragment that matches, so
    #   <tt>%r{pages/\d*/notes}</tt> might remove all notes. Make sure you
    #   don't use anchors in the regex (<tt>^</tt> or <tt>$</tt>) because
    #   the actual filename matched looks like
    #   <tt>./cache/filename/path.cache</tt>. 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 <tt>delete_matched</tt>, for Regexp keys).
    def expire_fragment(key, options = nil)
      return unless cache_configured?
      key = combined_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

    def instrument_fragment_cache(name, key) # :nodoc:
      ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}", instrument_payload(key)) { yield }
    end
  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/abstract_controller/caching/fragments.rb, line 109
      def read_fragment(key, options = nil)
        return unless cache_configured?

        key = combined_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

      # Check if a cached fragment from the location signified by
      # +key+ exists (see +expire_fragment+ for acceptable formats).
      def fragment_exist?(key, options = nil)
        return unless cache_configured?
        key = combined_fragment_cache_key(key)

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

      # Removes fragments from the cache.
      #
      # +key+ can take one of three forms:
      #
      # * String - This would normally take the form of a path, like
      #   <tt>pages/45/notes</tt>.
      # * Hash - Treated as an implicit call to +url_for+, like
      #   <tt>{ controller: 'pages', action: 'notes', id: 45}</tt>
      # * Regexp - Will remove any fragment that matches, so
      #   <tt>%r{pages/\d*/notes}</tt> might remove all notes. Make sure you
      #   don't use anchors in the regex (<tt>^</tt> or <tt>$</tt>) because
      #   the actual filename matched looks like
      #   <tt>./cache/filename/path.cache</tt>. 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 <tt>delete_matched</tt>, for Regexp keys).
      def expire_fragment(key, options = nil)
        return unless cache_configured?
        key = combined_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

      def instrument_fragment_cache(name, key) # :nodoc:
        ActiveSupport::Notifications.instrument("#{name}.#{instrument_name}", instrument_payload(key)) { yield }
      end
    end
  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/abstract_controller/caching/fragments.rb, line 96
def write_fragment(key, content, options = nil)
  return content unless cache_configured?

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