memcached as an Object store in Rails

Question

I am using Memcached as an Object Store with my Rails application where I store search results which are User objects in memcached

Now when I fetch the data out I get the Memcached Undefined Class/Module Error. I found a solution for this problem in this blog

http://www.philsergi.com/2007/06/rails-memcached-undefinded-classmodule.html

 before_filter :preload_models
  def preload_models
    Model1
    Model2
  end

which recommends pre-loading the models before hand. I would like to know if there is a more elegant solution to this problem and are there any drawbacks in using the preloading technique.

Thanks in advance

Answer 1

I had this problem as well and I think i came up with a nice solution.

You can overwrite the fetch method and rescue the error and load the right constants.

module ActiveSupport
  module Cache
    class MemCacheStore
      # Fetching the entry from memcached
      # For some reason sometimes the classes are undefined
      #   First rescue: trying to constantize the class and try again.
      #   Second rescue, reload all the models
      #   Else raise the exception
      def fetch(key, options = {})
        retries = 2 
        begin
          super
        rescue ArgumentError, NameError => exc         
          if retries == 2
            if exc.message.match /undefined class\/module (.+)$/
              $1.constantize
            end
            retries -= 1
            retry          
          elsif retries == 1
            retries -= 1
            preload_models
            retry
          else 
            raise exc
          end
        end
      end

      private

      # There are errors sometimes like: undefined class module ClassName.
      # With this method we re-load every model
      def preload_models     
        #we need to reference the classes here so if coming from cache Marshal.load will find them     
        ActiveRecord::Base.connection.tables.each do |model|       
          begin       
            "#{model.classify}".constantize 
          rescue Exception       
          end     
        end       
      end
    end
  end
end

Answer 2

Ran across this today, managed to come up with a more terse solution that should work for all classes.

Rails.cache.instance_eval do
  def fetch(key, options = {}, rescue_and_require=true)
    super(key, options)

  rescue ArgumentError => ex
    if rescue_and_require && /^undefined class\/module (.+?)$/ =~ ex.message
      self.class.const_missing($1)
      fetch(key, options, false)
    else
      raise ex
    end
  end
end

Not sure why [MemCacheStore] is not having is [MemCacheStore.const_missing] method called and everything getting called in the normal “Rails-y” way. But, this should emulate that!

Cheers,

Chris