cached_account.rb

class CachedAccount < CachedDelegateClass(Recurly::Account)
  self.cache_key = :account_code

  cache_constructor :find

  cache(:billing_info)
  cache(:subscription) { subscriptions.live.first }
  cache(:add_ons)      { subscription.try(:add_ons).to_a }
  cache(:plan)         { subscription.try :plan }
  cache(:balance)      {
    BigDecimal('0.01') * invoices.past_due.map(&:total_in_cents).sum
  }
end

cached_delegate_class.rb

def CachedDelegateClass(superklass)
  klass = DelegateClass(superklass)

  klass.class_attribute :delegate_class
  klass.delegate_class = superklass

  klass.class_attribute :cache_key
  klass.cache_key = :uuid

  class << klass
    def cache_key_for *args
      args.unshift delegate_class.name.tableize
      args.map(&:to_s).join '/'
    end

    private

    def cache_constructor method_name
      define_singleton_method method_name do |*args|
        Rails.cache.fetch cache_key_for(*args) do
          new(delegate_class.send(method_name, *args))
        end
      end
    end

    def cache method_name, &block
      ivar = "@_#{method_name.to_s.gsub /\W/, ''}"
      define_method method_name do
        return instance_variable_get ivar if instance_variable_defined? ivar
        val = if block
          instance_eval(&block)
        else
          __getobj__.send(method_name)
        end
        instance_variable_set ivar, val
        recache
        val
      end
    end
  end

  klass.class_eval do
    def cache_key
      send self.class.cache_key
    end

    private

    def recache
      Rails.cache.write self.class.cache_key_for(cache_key), self
    end
  end

  klass
end