不保存多个/属于关联时的计数器缓存行为

Question

给出以下模型和架构：

create_table "posts", force: :cascade do |t|
  t.integer  "comments_count", default: 0, null: false
end

create_table "comments", force: :cascade do |t|
  t.integer  "post_id"
end

class Post
  has_many :comments
end

class Comment
  belongs_to :post, counter_cache: true
end

运行以下代码段会产生一些奇怪的行为：

post = Post.create
post.comments_count # 0
post.comments.count # 0

comment = Comment.create
5.times do 
  comment.reload
  comment.post = post # NOTE: not saved but executes UDPATE for counter cache
end

post.reload
post.comments_count # 5
post.comments.count # 0

任何修复方法，以便只在实际修改外键时修改计数器缓存（最好是在事务中）？

日志：

Comment Load (0.4ms)  SELECT  "comments".* FROM "comments" WHERE "comments"."id" = $1 LIMIT $2  [["id", 2], ["LIMIT", 1]]
Post Load (0.2ms)  SELECT  "posts".* FROM "posts" ORDER BY "posts"."id" DESC LIMIT $1  [["LIMIT", 1]]
SQL (6.2ms)  UPDATE "posts" SET "comments_count" = COALESCE("comments_count", 0) + 1 WHERE "posts"."id" = $1  [["id", 1]]