module ActiveRecord::ConnectionAdapters::QueryCache

def cache
  old, @query_cache_enabled = @query_cache_enabled, true
  @query_cache ||= {}
  yield
ensure
  clear_query_cache
  @query_cache_enabled = old
end

def cache_sql(sql)
  result =
    if @query_cache.has_key?(sql)
      log_info(sql, "CACHE", 0.0)
      @query_cache[sql]
    else
      @query_cache[sql] = yield
    end
  if Array === result
    result.collect { |row| row.dup }
  else
    result.duplicable? ? result.dup : result
  end
rescue TypeError
  result
end

def clear_query_cache
  @query_cache.clear if @query_cache
end

def columns_with_query_cache(*args)
  if @query_cache_enabled
    @query_cache["SHOW FIELDS FROM #{args.first}"] ||= columns_without_query_cache(*args)
  else
    columns_without_query_cache(*args)
  end
end

def dirties_query_cache(base, *method_names)
  method_names.each do |method_name|
    base.class_eval <<-end_code, __FILE__, __LINE__ + 1
      def #{method_name}_with_query_dirty(*args)        # def update_with_query_dirty(*args)
        clear_query_cache if @query_cache_enabled       #   clear_query_cache if @query_cache_enabled
        #{method_name}_without_query_dirty(*args)       #   update_without_query_dirty(*args)
      end                                               # end
                                                        #
      alias_method_chain :#{method_name}, :query_dirty  # alias_method_chain :update, :query_dirty
    end_code
  end
end

def included(base)
  base.class_eval do
    alias_method_chain :columns, :query_cache
    alias_method_chain :select_all, :query_cache
  end
  dirties_query_cache base, :insert, :update, :delete
end

def select_all_with_query_cache(*args)
  if @query_cache_enabled
    cache_sql(args.first) { select_all_without_query_cache(*args) }
  else
    select_all_without_query_cache(*args)
  end
end

def uncached
  old, @query_cache_enabled = @query_cache_enabled, false
  yield
ensure
  @query_cache_enabled = old
end