Rails4 で dalli が標準の MemcacheClient になっちゃった対策。

legacy_mem_cache_store.rb

# Add legacy memcache session store for using KyotoTycoon with memcached protocol.
# Sourced from Rails 3.2
#  => https://github.com/rails/rails/blob/3-2-stable/actionpack/lib/action_dispatch/middleware/session/mem_cache_store.rb
#
require 'action_dispatch/middleware/session/abstract_store'
require 'rack/session/memcache'

module ActionDispatch
  module Session
    class LegacyMemCacheStore < Rack::Session::Memcache
      include Compatibility
      include StaleSessionCheck
      include SessionObject

      def initialize(app, options = {})
        require 'memcache'
        options[:expire_after] ||= options[:expires]
        super
      end
    end
  end
end