Q:

How can I prevent the odd insertion of :@new_record when cache hits in Dalli?

I'm implementing caching on Heroku using Dalli and Memcachier, and when I get a cache hit it's adding a Symbol to the returned array (or hash, in a different example). This Symbol, :@new_record, has nothing that I know of to do with the results I'm returning, is inserted on different types of cache fetches, and I haven't been able to find someone else on SO/Google with this issue. A codebase-wide find returns no results for "new_record". 
This addition has happened in two places where I've tried to add caching. I don't know how to prevent it from happening in the first place, but I dodge its effects in this particular use by checking the class of each record in the returned set, and not rendering a Supplier's profile if it's not a Supplier. This is an ugly solution, and I'd like to solve the problem at the source.
Please let me know if you have any additional questions / need me to post more information, and thanks for your help.

The method I'm using caching in:
  #this will be slow, need to store it somewhere
  def self.set_for_index(index_name)
    guide = INDEX_HOLDER[index_name]
    return false if guide.nil?
    haves, have_nots, countries = guide[0], guide[1], guide[2]
    holder = []
    supplier_set = Rails.cache.fetch("set_for_index_#{index_name}", :expires_in => 24.hours) {
      Supplier.find_each do |s|
        if (
            s.profile_visible and
            (countries == [] or (s.address and countries.include?(s.address.country))) and
            !(haves.map{ |h| s.has_tag?(Tag.find_by_name(h).id) }.include?(false)) and
            !(have_nots.map{ |h| s.has_tag?(Tag.find_by_name(h).id) }.include?(true))
          )
            holder << s
        end
      end
      holder
    }
    return supplier_set
  end

I'm screening out the :@new_record right now using the last line of this code: 
def self.visible_profiles_sorted(index_name)
      profiles = Supplier.set_for_index(index_name)

      answer = {}

      if !(profiles.nil? or profiles == [])
        profiles.each do |s|  
          #block odd error where cache appends a :@new_record after the last result
          next if !s.is_a?(Supplier) or s.address.nil? or s.address.country.nil?
...

The error, if I don't screen out the @new_record [this is not from cutting the line from the above code, but rather from another place I tried to use caching where a class-based test wasn't feasible]:
2013-08-28T20:50:00.446550+00:00 heroku[web.1]: State changed from starting to up
2013-08-28T20:50:06.774001+00:00 app[web.1]: Dalli/SASL authenticating as af5c2c
2013-08-28T20:50:06.777925+00:00 app[web.1]: Dalli/SASL: af5c2c
2013-08-28T20:50:06.807129+00:00 app[web.1]: Started GET "/suppliers" for 70.36.146.74 at 2013-08-28 20:50:06 +0000
2013-08-28T20:50:10.671553+00:00 app[web.1]: 
2013-08-28T20:50:10.671553+00:00 app[web.1]:     37:                                    <td>
2013-08-28T20:50:10.671553+00:00 app[web.1]: ActionView::Template::Error (undefined method `name' for :@new_record:Symbol):
2013-08-28T20:50:10.671553+00:00 app[web.1]:     39:                                            <%= link_to s.name, supplier_profile_path(s.name_for_link) %>
2013-08-28T20:50:10.671553+00:00 app[web.1]:     40:                                            <%= image_tag 

application.rb's relevant bit:
module Partreach
  class Application < Rails::Application

    config.cache_store = :dalli_store

production.rb's relevant bit:
Partreach::Application.configure do
  # Settings specified here will take precedence over those in config/application.rb

  # Code is not reloaded between requests
  config.cache_classes = true

  # Full error reports are disabled and caching is turned on
  config.consider_all_requests_local       = false
  config.action_controller.perform_caching = true

Gemfile's relevant bit:
gem 'dalli', '2.6.4' #https://devcenter.heroku.com/articles/building-a-rails-3-application-with-memcache
gem 'memcachier', '0.0.2' #https://devcenter.heroku.com/articles/building-a-rails-3-application-with-memcache

A:

I previously ran into the same thing.  Its even listed on on the issues page for dalli -- but it's not limited to it.
You can find more info here:
https://github.com/mperham/dalli/issues/250
And on the rails issues page too:
https://github.com/rails/rails/issues/8020
And this workaround resolved it for me:
https://github.com/rails/rails/issues/10322#issuecomment-16913855

