Override cache key and touch methods in active record when we cannot update the last modified / updated at column in database. We keep track of the changes in memcache.

override_cache_key.rb

module CacheKeyMonkeyPatch
  module ActiveRecord
    module Integration

      # Returns a cache key that can be used to identify this record.
      #
      # ==== Examples
      #
      #   Product.new.cache_key     # => "products/new"
      #   Product.find(5).cache_key # => "products/5" (updated_at / last_modified_time not available)
      #   Person.find(5).cache_key  # => "people/5-20140227181414671756000" (updated_at / last_modified_time available)

      def cache_key
        last_update_timestamp = Rails.cache.read(object_key)
        return "#{self.class.model_name.cache_key}/#{id}-#{last_update_timestamp}" if last_update_timestamp

        case
          when new_record?
            cached_cache_key = "#{self.class.model_name.cache_key}/new"
          when timestamp = (self[:updated_at] || self.respond_to?(:last_modified_time) && self.last_modified_time)
            timestamp = timestamp.to_s(:nsec)
            Rails.logger.info "\e[33m READ: #{object_key} from nil to #{timestamp}"
            Rails.cache.write(object_key, timestamp)
            cached_cache_key = "#{self.class.model_name.cache_key}/#{id}-#{timestamp}"
          else
            cached_cache_key = "#{self.class.model_name.cache_key}/#{id}"
        end

        cached_cache_key
      end
    end
  end
end

ActiveRecord::Base.class_eval do
  include CacheKeyMonkeyPatch::ActiveRecord::Integration
end

module ActiveRecord
  class Base
    def update_cache_key_for_associated_belongs_to_models(event)
      belongs_to_associations_to_touch = self.class.reflect_on_all_associations(:belongs_to).select { |assoc| assoc.options[:touch] }
      belongs_to_associations_to_touch.each do |belongs_to_assoc|
        associated_object = self.send(belongs_to_assoc.name)
        associated_object.touch("PROPAGATE from #{self.class.to_s}") if associated_object
      end
    end

    def touch(event = nil)
      event ||= 'CREATE'
      cached_timestamp = Rails.cache.fetch(object_key) || 'nil'
      new_timestamp = Time.zone.now.to_s(:nsec)

      Rails.logger.info "\e[33m #{event} #{object_key} from #{cached_timestamp} to #{new_timestamp}"
      Rails.cache.write(object_key, new_timestamp)
      update_cache_key_for_associated_belongs_to_models(event)
    end

    def object_key
      "cache_key_#{self.class.model_name.i18n_key}_#{self.id}"
    end
  end
end