Expirable Concern for Rails

expirable.rb

# Expirable is a module that lets you easily cache
# groups of records, either across an entire record:
#   cache(Snippet) => cache(Snippet.cache_key)
# or, across a scope:
#   cache(page.blocks) => cache(page.blocks.cache_key)
#
# Why not just use `maximum(:updated_at)`?
# Because it requires 2 queries: the total count,
# and the updated_at timestamp

# This method requires 0 queries, because we fetch
# everything via Memcached, then nuke it based on the
# after_save callbacks.
#
# Use this method when your data rarely, if ever, changes.
# This the regular updated_at timestamp when your data changes
# frequently, because the extra after_save callback slows your
# writes down.
module Concerns::Expirable
  extend ActiveSupport::Concern

  included do
    after_destroy :clear_expirable_cache_key
    after_save    :clear_expirable_cache_key
    after_touch   :clear_expirable_cache_key
  end

  module ClassMethods
    def cache_key
      Rails.cache.fetch(expirable_cache_key) do
        "#{count}-#{last_modified}"
      end
    end
    alias etag cache_key

    def expirable_cache_key
      Array[name, :cache_key]
    end

    def last_modified
      maximum(:updated_at).utc
    end
  end

protected

  def clear_expirable_cache_key
    Rails.cache.delete self.class.expirable_cache_key
  end
end