Collection Cache Key

给 Relation 集合也可以有 cache_key，例如：

blogs = Blog.where("id > 2");

blogs.collection_cache_key
   (0.2ms)  SELECT COUNT(*) AS size, MAX("blogs"."updated_at") AS timestamp
   FROM "blogs" WHERE (id > 2)
# => "blogs/query-9a5d6ea830ba519707a5aaf189b9a1b1-0"

ActiveRecord::Base#cache_key 封装了它，所以直接用 cache_key 即可。