渴望加载关联模型的最后一条记录

如何解决渴望加载关联模型的最后一条记录

我有一个模型 Invoice 其中 has_many Payments 和一个模型 Payment belongs_to Invoice。

Invoice.rb

class Invoice < ApplicationRecord
  has_many :payments,inverse_of: :invoice
end

Payment.rb

class Payment < ApplicationRecord
  belongs_to :invoice
end

我们每月批量导出Invoice数据，我们需要每个Invoice的最后一次付款。

在我们看来，我们目前对要导出的每个发票执行 Invoice.payments.last 一次，并且我被要求阻止 N+1 查询。

我尝试将以下关联添加到 Invoice 模型：

has_one     :last_approved_payment,-> { invoice.payments.approved.last },class_name: "Payment",inverse_of: :invoice

但我收到以下错误：

Invoice.first.last_approved_payment
  Invoice Load (0.4ms)  SELECT "invoices"."id","invoices"."user_id","invoices"."invoiceable_type","invoices"."invoiceable_id","invoices"."status","invoices"."number","invoices"."period","invoices"."external_id","invoices"."external_status","invoices"."created_at","invoices"."updated_at","invoices"."due_date","invoices"."Metadata","invoices"."rejection_code","invoices"."retry_attempt","invoices"."next_retry_date","invoices"."last_retry_date","invoices"."expire_date","invoices"."external_service","invoices"."external_type","invoices"."external_info","invoices"."account_id","invoices"."temp_due_date","invoices"."next_attempt_date","invoices"."last_attempt_date","invoices"."processing_stage","invoices"."amount_cents","invoices"."amount_currency","invoices"."desist_retrying" FROM "invoices" ORDER BY "invoices"."id" ASC LIMIT $1  [["LIMIT",1]]
  Payment Load (0.3ms)  SELECT "payments"."id","payments"."operation_id","payments"."status","payments"."status_detail","payments"."payment_type","payments"."external_reference","payments"."payer_email","payments"."date_created","payments"."date_approved","payments"."money_release_date","payments"."comment","payments"."created_at","payments"."updated_at","payments"."user_id","payments"."operation_type","payments"."reason","payments"."reasonable_type","payments"."reasonable_id","payments"."invoice_id","payments"."Metadata","payments"."account_id","payments"."amount_cents","payments"."amount_currency","payments"."payer_id_ciphertext" FROM "payments" LIMIT $1  [["LIMIT",11]]
  Payment Load (0.4ms)  SELECT "payments"."id",11]]
**NameError: undefined local variable or method `invoice' for #<Payment::ActiveRecord_Relation:0x000055b544ccf138>
from /home/vm/.rbenv/versions/2.6.3/lib/ruby/gems/2.6.0/gems/activerecord-6.0.3.6/lib/active_record/relation/delegation.rb:109:in `method_missing'**

解决方法

您无需为模型关系明确添加默认范围，您可以做的只是在检索发票时预加载 payments 表；

Invoice.includes(:payments)

尽管如果您只从两种模型中选择您真正需要的东西，IMO 会更好，但您可以通过使用 select 的子查询来实现：

Invoice.select(
  :id,"(SELECT p.id,FROM payments p
    WHERE p.invoice_id = invoices.id
    ORDER BY created_at DESC
    LIMIT 1) AS payment_id"
)