Ruby On Rails - 自引用关联：避免创建两次相同的友谊关系

Question

我有一个User 模型：

has_many :friendships, dependent: :destroy
has_many :friends, through: :friendships
has_many :inverse_friendships, class_name: "Friendship", foreign_key: "friend_id", dependent: :destroy
has_many :inverse_friends, through: :inverse_friendships, source: :user

还有一个Friendship 模型：

belongs_to :user
belongs_to :friend, class_name: "User"

Friendship 表同时具有user_id 和friend_id（user_id 是创建user 关系的id）。

我想添加一个validation，它不允许创建两次相同的friendship 关系（请看下面的示例）：

## first_user has id = 103
## second_user has id = 209
## I don't want to have:
Frienship<id = 1072, user_id = 103, friend_id = 209>
Frienship<id = 3022, user_id = 209, friend_id = 103>
## i.e, I don't want to store this relationship two times.

Answer 1

你应该写一个custom validator：

class Friendship < ActiveRecord::Base
  validate :friendship_validation

  private

  def friendship_validation
    if Friendship.where("(user_id=? AND friend_id=?) OR (user_id=? AND friend_id=?)", self.user_id, self.friend_id, self.friend_id, self.user_id).any?
      errors.add(:friendship, "friendship exists")
    end
  end
end