如何在 Rails 7 中预加载 STI 模型的关联？

Question

我的问题

想象一下我有那些模型：

class Absence << ApplicationRecord
  belongs_to :user
end

class Vacation << Absence
  belongs_to :vacation_contingent
end

class Illness << Absence; end

现在我想用

absences = Absence.where(user: xxx)

并遍历假期队伍

vacations = absences.select { |absence| absence.is_a?(Vacation)
vacations.each { |vacation| puts vacation.vacation_contingent.xxx }

现在我有 1 个针对这些缺席的数据库查询，每个 vacation_contingent 有 1 个 -> 坏

PS：我使用Absence.where而不是Vacation.where，因为我想用那些absences做其他事情。

我试过的

1. 当然

   Absence.where(user: xxx).includes(:vacation_contingent)
   # -> ActiveRecord::AssociationNotFoundError Exception: Association named 'vacation_contingent' was not found`
   

   vacations = Vactions.where(user: xxx).includes(:vacation_contingent)
   other_absences = Absence.where(user: xxx).where.not(type: 'Vacation')
   

   但这一个很丑陋，我有 1 个数据库查询比我想要的多，因为我正在获取 2 次缺席。

   3.

   absences = Absence.where(user: xxx)
   vacations = absences.select { |absence| absence.is_a?(Vacation)
   preloader = ActiveRecord::Associations::Preloader.new
   preloader.preload(vacations, :vacation_contingent)
   # -> ArgumentError Exception: missing keywords: :records, :associations
   # -> (The initializer changed)
   

   absences = Absence.where(user: xxx)
   vacations = absences.select { |absence| absence.is_a?(Vacation)
   preloader = ActiveRecord::Associations::Preloader.new(records: vacations, associations: %i[vacation_contingent])
   # -> This line does nothing on it's own
   preloader.call
   # -> This just executes SELECT "vacation_contingents".* FROM "vacation_contingents" vacation.size times
   preloader.preload
   # -> Does the same as .call
   # -> And this doesn't even preload anything. When executing
   vacations.first.vacation_contingent
   # -> then the database is being asked again
   

Answer 1

在我看来，解决方案 2 是您可以使用 ActiveRecord 做的最好的事情。

如果只需要一个请求，可以使用原始 SQL 来完成；就像是 ：

Absence.connection.select_all(%{SELECT * 
                                FROM absences
                                LEFT OUTER JOIN vacation_contingents ON absences.vacation_contingents_id = vacation_contingents.id
                                WHERE absences.user_id = ?", user_xxx.id})

它将返回 ActiveRecord::Result，每个 Absence 以及 Absence 和 VacationContingent 的列都有一行