我正在学习Ruby on Rails，目前正在努力实现多对多的关系。我有一个用户和一个拼车，可以由一个用户驾驶，由多个用户乘坐。因此，我尝试用has_and_belongs_to_many实现一个连接表carpools_users。我假设我的模型出了问题，但我不知道我到底做错了什么。
当前会抛出RuntimeError，因为我违反了carpools_users.carpool_id的非空约束。

移民

20230621132948_create_carpools.rb

class CreateCarpools < ActiveRecord::Migration[7.0]
  def change
    create_table :carpools do |t|
      t.references :driver, foreign_key: { to_table: 'users' }
      t.references :passenger, foreign_key: { to_table: 'users' }

      t.timestamps
    end
  end
end

20230621123159_create_users.rb

class CreateUsers < ActiveRecord::Migration[7.0]
  def change
    create_table :users do |t|
      t.string :name
      t.references :ridden_carpool, foreign_key: { to_table: 'carpools' }
      t.references :driven_carpool, foreign_key: { to_table: 'carpools' }

      t.timestamps
    end

    create_join_table :users, :carpools, :id => false
  end
end

型号

• 拼车.rb*

class Carpool < ApplicationRecord
  belongs_to :driver, :class_name => 'User', :foreign_key  => "driver_id"
  has_and_belongs_to_many :passenger, :class_name => 'User', :foreign_key  => "user_id"
end

• user.rb*

class User < ApplicationRecord
    has_many :driven_carpools, :class_name => "Carpool", :foreign_key  => "driven_carpool_id"
    has_and_belongs_to_many :ridden_carpools, :class_name => "Carpool", :foreign_key  => "carpool_id"
end