ruby-on-rails - rails 4 : counter_cache in has_many :through association with dependent: :destroy

Question

虽然已经有人问过类似的问题:

counter_cache with has_many :through

dependent => destroy on a "has_many through" association

has_many :through with counter_cache

他们都没有真正解决我的问题。

我有三个模型，有一个 has_many :through 关联:

class User < ActiveRecord::Base
  has_many :administrations
  has_many :calendars, through: :administrations
end

class Calendar < ActiveRecord::Base
  has_many :administrations
  has_many :users, through: :administrations
end

class Administration < ActiveRecord::Base
  belongs_to :user
  belongs_to :calendar
end

联接管理模型具有以下属性:

id
user_id
calendar_id
role

我要数多少 calendars每个 user有多少 users每个 calendar已。

我打算使用 counter_cache 如下:

class Administration < ActiveRecord::Base
  belongs_to :user, counter_cache: :count_of_calendars
  belongs_to :calendar, counter_cache: :count_of_users
end

(当然，还有将 :count_of_calendars 添加到 users 表和 :count_of_users 到 calendars 表的相应迁移。)

但是后来，我偶然发现了 this warning in Rails Guides :

4.1.2.4 :dependent

If you set the :dependent option to:

• :destroy, when the object is destroyed, destroy will be called on its associated objects.
• :delete, when the object is destroyed, all its associated objects will be deleted directly from the database without calling their destroy method.

You should not specify this option on a belongs_to association that is connected with a has_many association on the other class. Doing so can lead to orphaned records in your database.

因此，计算多少 calendars 是一个好习惯。每个 user有多少 users每个 calendar已？

Answer 1

那么，dependent: :destroy将销毁关联的记录，但不会更新 counter_cache ，因此您可能在 counter_cache 中计数错误.相反，您可以实现一个回调来销毁关联的记录，并更新您的 counter_cache .

class Calendar < ActiveRecord::Base

  has_many :administrations
  has_many :users, through: :administrations


  before_destroy :delete_dependents

  private
  def delete_dependents
    user_ids = self.user_ids
    User.delete_all(:calendar_id => self.id)
    user_ids.each do |u_id|
      Calendar.reset_counters u_id, :users
    end
  end
end

同样，为 User 实现此操作模型也是