Rails 4或5，stale或fresh_when方法，包含查询

Question

我正在使用Rails 5：

def index
  @posts = Post.all
  fresh_when(etag: @posts, last_modified: @posts.maximum(:updated_at))
end

这是Rails日志：

(1.0ms)  SELECT MAX("posts"."updated_at") FROM "posts"
(0.7ms)  SELECT COUNT(*) AS "size", MAX("posts"."updated_at") AS timestamp FROM "posts"

但如果我有这样的查询怎么办：

@posts = Post.includes(:comments).all

Rails的日志是：

Started GET "/posts"
Processing by PostsController#index as HTML
   (0.2ms)  SELECT MAX("posts"."updated_at") FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id"
   (0.1ms)  SELECT COUNT(*) AS "size", MAX("posts"."updated_at") AS timestamp FROM "posts"   

但任何评论。事实上，如果我修改该帖子的一个评论和＆＃34; updated_at＆＃34;列更改它不会更新查询。

如何？

没有＆＃34;包括（：评论）＆＃34;一切都很好。