Rails模型中的class_name foreign_key [英] class_name foreign_key in Rails model

问题描述

我最近遇到了这段代码。用户有很多答案。 ：class_name和：foreign_key的目的是什么？

I recently come across this code. User has many Answer. What is the purpose of the :class_name and :foreign_key ?

class Answer < ApplicationRecord
    belongs_to :user, :class_name => 'Question", :foreign_key => 'question_id'
end

推荐答案

这里的命名有点奇怪，但是：class_name 的目的是允许您使用与Rails期望的类不同的类。当您在模型上具有 belongs_to：user 时，Rails希望它指向名为 User 的父类。在您的示例中，Rails跳过了查找 User 类的过程，而是查找了 Question 模型。

The naming here is kind of strange, but the purpose of :class_name is to allow you to use a class that is different from the one Rails expects. When you have a belongs_to :user on a model, Rails would expect that to point to a parent class called User. In your example, Rails skips looking for a User class and instead looks to the Question model.

但是，最常见的用法是使用非默认关联名称比使用默认名称更有意义，因此更合适的示例是当您使用用户模型和竞赛模型，其中每个竞赛都有一个用户作为获胜者，您可以每个竞赛属于 User ，但这并不具有表现力，相反，您可能希望将该关系称为 winner ：

The most common usage of this, though, is when a non-default association name makes more sense than the default. So a more apt example is when you have a User model and Competition model wherein each competition has one user as a winner. You could have each Competition belong to a User, but that wouldn't be as expressive. Instead you may want to have the relationship be referred to as winner:

class User < ActiveRecord::Base
  has_many :winners, class_name: "Competition", foreign_key: "competition_id"
end

class Competition < ActiveRecord::Base
  belongs_to :winner, class_name: "User", foreign_key: "winner_id"
end

这使您可以将用户称为赢家：

This allows you to refer to users as winners:

competition = Competition.first
competition.winner

这要多得多比您编写 competition.user 更具表现力。

This is a lot more expressive than if you were to write competition.user.

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