ruby-on-rails - 确保 has_many :through association is unique on creation

Question

如果您在记录创建时保存一个 has_many :through 关联，您如何确保该关联具有唯一对象。唯一性由一组自定义属性定义。

考虑:

 class User < ActiveRecord::Base
   has_many :user_roles
   has_many :roles, through: :user_roles

   before_validation :ensure_unique_roles

   private
   def ensure_unique_roles
      # I thought the following would work:
      self.roles = self.roles.to_a.uniq{|r| "#{r.project_id}-#{r.role_id}" }
      # but the above results in duplicate, and is also kind of wonky because it goes through ActiveRecord assignment operator for an association (which is likely the cause of it not working correctly)

     # I tried also:
     self.user_roles = []
    self.roles = self.roles.to_a.uniq{|r| "#{r.project_id}-#{r.role_id}" }

     # but this is also wonky because it clears out the user roles which may have auxiliary data associated with them
   end
 end

根据关联的任意条件验证 user_roles 和角色是否唯一的最佳方法是什么？

Answer 1

最好的方法是在 user_roles 上创建一个唯一的多列索引，尤其是当您使用关系数据库时。

add_index :user_roles, [:user_id, :role_id], 唯一: true

然后在角色添加失败时优雅地处理:

class User < ActiveRecord::Base
  def try_add_unique_role(role)
    self.roles << role
  rescue WhateverYourDbUniqueIndexExceptionIs
    # handle gracefully somehow
    # (return false, raise your own application exception, etc, etc)
  end
end

关系数据库旨在保证引用完整性，因此请将其用于此目的。任何 ruby​​/rails-only 解决方案都会有竞争条件和/或非常低效。

如果您想提供用户友好的消息传递并勾选“以防万一”，请继续检查:

already_has_role = UserRole.exists?(user: user, role: prospective_role_additions)

不过，当您尝试持续添加角色时，您仍然必须处理潜在的异常。