class_name foreign_key in Rails model

问题:

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

回答1:

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.

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

This allows you to refer to users as winners:

competition = Competition.first
competition.winner

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