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我有Gallery和Post模型，has_and_belongs_to_many两者之间有关系。模型上有一个posts_count列Gallery来跟踪给定画廊中有多少帖子。它工作正常，但它执行的 DB 命中次数超出了必要的范围。

如果我做类似的事情，gallery.posts.clear那么我会为每个被删除的帖子进行数据库操作：

(0.3ms)  BEGIN
(0.9ms)  DELETE FROM "galleries_posts" WHERE "galleries_posts"."gallery_id" = 6 AND "galleries_posts"."post_id" IN (8, 6, 5, ..., 39, 40, 42, 41, 41, 44, 45, 47, 50, 49, 51)
(0.7ms)  UPDATE "galleries" SET "posts_count" = 54, "updated_at" = '2013-10-22 22:56:32.048238' WHERE "galleries"."id" = 6
(0.4ms)  UPDATE "galleries" SET "posts_count" = 53, "updated_at" = '2013-10-22 22:56:32.050582' WHERE "galleries"."id" = 6
(0.3ms)  UPDATE "galleries" SET "posts_count" = 52, "updated_at" = '2013-10-22 22:56:32.052247' WHERE "galleries"."id" = 6
...
(0.3ms)  UPDATE "galleries" SET "posts_count" = 3, "updated_at" = '2013-10-22 22:56:32.055607' WHERE "galleries"."id" = 6
(0.3ms)  UPDATE "galleries" SET "posts_count" = 2, "updated_at" = '2013-10-22 22:56:32.084547' WHERE "galleries"."id" = 6
(0.3ms)  UPDATE "galleries" SET "posts_count" = 1, "updated_at" = '2013-10-22 22:56:32.085752' WHERE "galleries"."id" = 6
(0.3ms)  UPDATE "galleries" SET "posts_count" = 0, "updated_at" = '2013-10-22 22:56:32.086861' WHERE "galleries"."id" = 6
(3.6ms)  COMMIT

这相当于n更新计数的数据库操作，其中n是添加或删除的帖子数。

posts_count一旦添加或删除了所有帖子，有没有办法维持计数，而不是依赖于仅更新画廊一次的操作？

我当前的方法在after_add/上运行after_remove：

class Gallery < ActiveRecord::Base
  ...
  has_and_belongs_to_many :posts, after_add: :increment_posts_count, 
                                  after_remove: :decrement_posts_count
  ...
  private
    def increment_posts_count(post)
      self.increment!(:posts_count)
    end

    def decrement_posts_count(post)
      self.decrement!(:posts_count)
    end
end

只有在整个操作完成后才能运行任何钩子？

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