在 through 模型中使用作用域条件定义“has_many through：关联”答案

【问题标题】：Defining "has_many through: assocation" with scoped conditionals in the through model在 through 模型中使用作用域条件定义“has_many through：关联”
【发布时间】：2016-08-19 23:11:02
【问题描述】：

我有一个看起来像这样的架构：

create_table "customers" do |t|
  t.integer "customer_number"
end

create_table "past_payments" do |t|
  t.integer  "customer_number"

  t.datetime "transaction_date"
  t.integer  "arbitrary_sequence_number"
end

create_table "payment_details" do |t|
  t.datetime "transaction_date"
  t.integer  "arbitrary_sequence_number"
end

TL;架构中的 DR - 客户通过主键/外键与过去付款相关联。并且当它们的 transaction_date 和 optional_sequence_number 相等时，PastPayment 与单个 PaymentDetail 相关联。 Payments 和 Details 没有正式的主键/外键关系。

这给了我以下 ActiveRecord 模型：

class Customer < ActiveRecord::Base
  has_many :past_payments, foreign_key: :customer_number, primary_key: :customer_number

  has_many :payment_details, through: :past_payments # unfortunately, broken ????
end

class PastPayment < ActiveRecord::Base
  has_one :payment_detail, ->(past_payment) {
    where(arbitrary_sequence_number: past_payment.arbitrary_sequence_number)
  }, foreign_key: :transaction_date, primary_key: :transaction_date
end

由于Customer has_many :past_payments 和PastPayment has_one :payment_detail，我认为有一个关联可以定义为Customer has_many :payment_details, through: :past_payments，但由于has_one :payment_detail 上定义的范围，我无法使其工作协会。

具体来说，调用Customer.payment_details 会引发NoMethodError: undefined method 'arbitrary_sequence_number' for #<Customer:0x2i8asdf3>。因此，与 PastPayment 相比，客户似乎正在传递到我的范围。

是否可以在 Customer 上定义 has_many :payment_details 关联？我是不是做错了什么？

为了清楚起见，我希望能够说 Customer.where(some_conditions).includes(:payment_details) 并只执行两个查询，所以如果有办法在没有关联的情况下完成它，我愿意接受。

注意：我无法更改此数据库。这是一个其他应用程序写入的数据库，我需要从中读取。

与我的问题无关，这是我目前正在使用的解决方法。如果无法正确使用关联，我很乐意批评此解决方案：

class Customer < ActiveRecord::Base
  attr_writer :payment_details

  def payment_details
    @payment_details ||= Array(self).with_payment_details.payment_details
  end

  module InjectingPaymentData
    def with_payment_details
      results = self.to_a
      return self unless results.first.is_a?(Customer)

      user_ids = results.collect(&:id)

      # i've omitted the details of the query, but the idea is at the end of it
      # we have a hash with the customer_number as a key pointing to an array
      # of PaymentDetail objects
      payment_details = PaymentDetails.joins().where().group_by(&:customer_number)

      results.each do |customer|
        customer.payment_details = Array(payment_details[customer.customer_number])
      end
    end
  end
end

ActiveRecord::Relation.send(:include, Customer::InjectingPaymentData)
Array.send(:include, Customer::InjectingPaymentData)

这样我就可以用最少的查询完成以下操作：

@customers = Customer.where(id: 0..1000).with_payment_details
@customers.each { |c| do_something_with_those_payment_details }

这种方法有问题吗？

【问题讨论】：

标签： ruby-on-rails ruby ruby-on-rails-4 activerecord associations

【解决方案1】：

您可以利用此 gem 更好地处理 ActiveRecord 中的“复合主键”：https://github.com/composite-primary-keys/composite_primary_keys

class Customer < ActiveRecord::Base
  has_many :past_payments, foreign_key: :customer_number, primary_key: :customer_number

  has_many :payment_details, through: :past_payments # this works now
end

class PastPayment < ActiveRecord::Base
  has_one :payment_detail, foreign_key: [:transaction_date, :arbitrary_sequence_number],
                           primary_key: [:transaction_date, :arbitrary_sequence_number]
end

【讨论】：