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

none 其中实际上解决了我的问题。

我有三个模型，有一个 has_many :through association :

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

加入管理模型具有以下属性：

id
user_id
calendar_id
role

我想数一数每个user有多少个calendars，每个calendar有多少个users。

我本来打算 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，相应的迁移将 :count_of_calendars 添加到 users table 并将 :count_of_users 添加到 calendars table.)

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，计算每个 user 有多少 calendars 以及每个 calendar 有多少 users 是一个好的做法吗？

Answer 1

嗯，dependent: :destroy会破坏相关记录，但不会更新counter_cache，所以你可能在counter_cache中记错了。相反，您可以实施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，也为 User 模型实施此方法

Rails 4: counter_cache in has_many :through association with dependent: :destroy

Rails 4: counter_cache in has_many :through association with dependent: :destroy

ruby-on-rails

has-many-through

counter-cache

ruby-on-rails-4