如何为组合字段的唯一性建模此复杂验证

A link有两个components:componenta_id和componentb_id.为此,Link我在模型文件中有:

belongs_to :componenta, class_name: "Component"
belongs_to :componentb, class_name: "Component"

validates :componenta_id, presence: true
validates :componentb_id, presence: true
validates :componenta_id, uniqueness: { scope: :componentb_id }
validates :componentb_id, uniqueness: { scope: :componenta_id }

并在迁移文件中:

create_table :links do |t|
  t.integer  :componenta_id, null: false 
  t.integer  :componentb_id, null: false
  ...
end
add_index :links, :componenta_id
add_index :links, :componentb_id
add_index :links, [:componenta_id, :componentb_id], unique: true

问题:这一切都有效.现在,无论他们的顺序如何,我都想要它们的组合componanta和componentb独特性.因此无论哪个组件是哪个组件(componenta哪个是componentb相同的链接;两个相同组件之间的链接).因此,不应允许下面的两个记录,因为它们代表相同的链接,因此不是唯一的:

componenta_id = 1; componentb_id = 2

componenta_id = 2; componentb_id = 1

如何创建此唯一性验证？我有模型验证工作(见下文),但想知道是否以及如何在迁移/数据库级别添加验证...？

模型验证
我使用以下代码进行模型验证:

before_save :order_links
validates :componenta_id, uniqueness: { scope: :componentb_id }

private
  def order_links
    if componenta_id > componentb_id
      compb = componentb_id
      compa = componenta_id
      self.componenta_id = compb
      self.componentb_id = compa
    end
  end

以下测试证实了上述工作:

  1. test "combination of two links should be unique" do
  2.   assert @link1.valid?
  3.   assert @link2.valid?
  4.   @link1.componenta_id = 3     #@link2 already has combination 3-4
  5.   @link1.componentb_id = 4
  6.   assert_not @link1.valid?
  7.   @link1.componenta_id = 4
  8.   @link1.componentb_id = 3
  9.   assert_raises ActiveRecord::RecordNotUnique do
  10.    @link1.save
  11.  end
  12.end

迁移/数据库验证:
作为额外的安全级别,还有一种方法可以在数据库级别上对此进行验证吗？否则,仍然可以将以下两个记录写入数据库:componenta_id = 1 ; componentb_id = 2以及componenta_id = 2 ; componentb_id = 1.