使用两个外键建立关联时出错答案

【问题标题】：Error building an association with two foreign keys使用两个外键建立关联时出错
【发布时间】：2018-07-30 02:24:46
【问题描述】：

我有一个用户和推荐模型。用户有几种帐户类型：学生、管理员、推荐人等。学生用户可以分配推荐人来提交推荐。我没有使用 Rolify 所以

我目前有：

class Recommendation < ApplicationRecord
  belongs_to :assigner, class_name: "User", foreign_key: "user_id"
  belongs_to :writer,   class_name: "User", foreign_key: "writer_id"
end

class User < ApplicationRecord
  has_many :recommendations_via_assigning, class_name: "Recommendation", foreign_key: :assigner_id
  has_many :recommendations_via_writing,   class_name: "Recommendation", foreign_key: :writer_id
end

class CreateRecommendations < ActiveRecord::Migration[5.1]
  def change
    create_table :recommendations do |t|
      t.integer :writer_id, foreign_key: true
      t.integer :assigner_id, foreign_key: true
      t.text    :text

      t.timestamps
    end

    add_index :recommendations, :writer_id
    add_index :recommendations, :assigner_id
  end
end

控制器

 def new
   @recommendation = current_user.recommendations_via_assigning.build
 end

 def create
   @recommendation = current_user.recommendations_via_assigning.build(recommendation_request_params)
   if @recommendation.save
     @recommendation.send_invitation_email
     flash[:info] = "An email with submission instructions has been sent to the recommender you have invited."
     redirect_to user_url
   else
     render 'new'
   end
 end

 private
    def recommendation_request_params
     params.require(:recommendation).permit(:assigner_id, :first_name, :last_name, :email, :relationship)
   end

   def recommendation_content_params
     params.require(:recommendation).permit(:writer_id, :first_name, :last_name, :email, :text)
   end

我无法保存推荐，因为它需要 writer_id，在提交 Recommendation#create 时不会知道该推荐。需要做的是向提供的电子邮件地址发出一封电子邮件，邀请教师创建一个帐户，当该帐户注册时，通过 id 和教师帐户的 writer_id 进行推荐。

如何修改现有代码，以便在没有 writer_id 的情况下通过推荐#create？

【问题讨论】：

标签： ruby-on-rails associations

【解决方案1】：

首先正确声明外键：

class CreateRecommendations < ActiveRecord::Migration[5.0]
  def change
    create_table :recommendations do |t|
      t.belongs_to :writer, foreign_key: { to_table: :users }
      t.belongs_to :assigner, foreign_key: { to_table: :users }

      t.timestamps
    end
  end
end

当不能从列名派生时，需要手动指定外键约束的引用表。

您还需要将关联设置为可选以删除验证：

class Recommendation < ApplicationRecord
  belongs_to :assigner, class_name: "User", 
                        foreign_key: "user_id"
  belongs_to :writer,   class_name: "User", 
                        foreign_key: "writer_id",
                        optional: true
end

【讨论】：

【解决方案2】：

Read here

您需要将belongs_to 关联设为可选：

belongs_to :writer, class_name: "User", foreign_key: "writer_id", optional: true

【讨论】：