Simple ActiveRecord query cache

application_controller.rb

class ApplicationController < ActionController::Base
  ...
    
  around_action :use_request_cache, if: Proc.new {|c| request.get? }

  private

  def use_request_cache
    RequestCache.with_cache do
      yield
    end
  end
  
  ...
end

confing_initializers_request_cache.rb

# Monkey-patch AR relation to use RequestsCache to fetch records
module ActiveRecord
  class Relation
    def records # :nodoc:
      RequestCache.fetch to_sql do
        load
        @records
      end
    end
  end
end

class ActiveRecord::ConnectionAdapters::PostgreSQLAdapter
  def clear_query_cache
    RequestCache.clear
    super
  end
end

request_cache.rb

module RequestCache
  def self.with_cache(request_cache = {}, &block)
    Thread.current[:request_cache] = request_cache
    yield
  ensure
    RequestCache.clear
  end

  def self.clear
    Rails.logger.debug "RequestCache: Cleared!"
    Thread.current[:request_cache] = nil
  end

  def self.fetch key
    if Thread.current[:request_cache].nil?
      Rails.logger.debug "RequestCache: Disabled: #{key}"
      return yield
    else
      val = Thread.current[:request_cache][key]
      if val
        Rails.logger.debug "RequestCache: Hit: #{key}"
        if val == :__nil__
          return nil
        else
          return val
        end
      else
        Rails.logger.debug "RequestCache: Miss: #{key}"
        val = yield
        Thread.current[:request_cache][key] = val || :__nil__
        return val
      end
    end
  end
end