如何在rails中实现“独特的多对多自我联合”？

class User < ApplicationRecord
  has_many :friendships
  has_many :friends, through: :friendships, source: :user

  # 确保好友关系是双向的
  after_create :add_self_as_friend

  private

  def add_self_as_friend
    self.friends << self unless self.friends.include?(self)
  end
end

class Friendship < ApplicationRecord
  belongs_to :user
  belongs_to :friend, class_name: 'User', foreign_key: 'friend_id'

  # 确保关系的唯一性
  validates :user_id, uniqueness: { scope: :friend_id }
end

rails generate migration CreateFriendships user:references friend:references
rake db:migrate

class AddUniqueConstraintToFriendships < ActiveRecord::Migration[6.1]
  def change
    add_index :friendships, [:user_id, :friend_id], unique: true
  end
end
rake db:migrate

def add_friend(friend)
  unless self.friends.include?(friend)
    self.friendships.create(friend: friend)
  end
end

class User < ApplicationRecord
  has_many :friendships
  has_many :friends, through: :friendships, source: :user

  def friends_of_friends
    friends.joins(:friendships).where.not(friendships: { user_id: self.id }).distinct
  end
end