has_many :通过 counter_cache [英] has_many :through with counter_cache

问题描述

我的理解是，在定义 :counter_cache 选项时，它将在包含belongs_to 声明的模型上指定.所以我有点不确定在通过关联使用 has_may 时如何处理这个问题(因为我相信在这种情况下不使用belongs_to 声明):

It is my understanding that when defining a :counter_cache option it is to be specified on the model that includes the belongs_to declaration. So I am a little unsure of how to handle this when working with a has_may through association (as I believe that a belongs_to declaration is not used in this scenario):

class Physician < ActiveRecord::Base
  has_many :appointments
  has_many :patients, :through => :appointments
end

class Appointment < ActiveRecord::Base
  belongs_to :physician, :counter_cache => appointment_count
end

class Patient < ActiveRecord::Base
end

我希望使用 :counter_cache 选项来更有效地查找属于医生的患者数量.

I wish to use the :counter_cache option to make finding the number of Patients belonging to a Physician more efficient.

myPhysician.patients.count

仅供参考:Rails 3.1

FYI: Rails 3.1

干杯

推荐答案

我不确定你想要什么样的关系.该示例类似于Rails 指南

I'm not sure what kind of relationship you want. That example is similar to the one in the Rails Guide

class Physician < ActiveRecord::Base
  has_many :appointments
  has_many :patients, :through => :appointments
end

class Appointment < ActiveRecord::Base
  belongs_to :physician
  belongs_to :patient
end

class Patient < ActiveRecord::Base
  has_many :appointments
  has_many :physicians, :through => :appointments
end

• 一个Physician有很多预约，有很多Patients
• 一个预约属于(有一个)Physician和一个Patient
• 一个病人有很多预约和很多医师.
• A Physician has many Appointments, and has many Patients
• An Appoinment belongs to (has one) Physician and one Patient
• a Patient has many Appointments and many Physicians.

关于 :counter_cache 选项，根据 belongs_to doc:如果您想要属于 Physician 的 Patients 数量，您需要:

Regarding the :counter_cache option, according to the belongs_to doc: If you want the number of Patients belonging to a Physician you would need:

class Appointment < ActiveRecord::Base
  belongs_to :physician, :counter_cache => :patient_count
  belongs_to :patient
end

并且您需要编写迁移以将patient_count列添加到Phyisicans表中.

And you need to write a migration to add the patient_count column to the Phyisicans table.

但是，对于 has_many through 关系，Rails 3.1 似乎会自动检测 counter_cache 列，因此您不必指定它(删除 :counter_cache => :patient_count).如果你指定它，你的计数器会增加两个(这很奇怪).

However, for has_many through relationships Rails 3.1 seems to automatically detect the counter_cache column, so you don't have to specify it (remove :counter_cache => :patient_count). If you do specify it your counter will go up by two (this is very weird).

顺便说一下，R​​ails 3.1 中的 :counter_cache 选项似乎一些问题，如下所述:

By the way, there seems to be some problems with :counter_cache option in Rails 3.1, as reported here:

• https://github.com/rails/rails/issues/3903
• https://github.com/rails/rails/issues/3085

考虑到所有这些，也许最好的办法是使用回调编写自己的计数机制.

With all of that in mind, maybe your best bet is to write your own count mechanism using callbacks.

希望有帮助:)

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