rails5:当我对模型进行验证时,如何在数据库级别测试惟一性?

我有以下联接表:

class ContactFormUsership < ApplicationRecord
  belongs_to :user
  belongs_to :contact_form

  validates :user_id, presence: true, uniqueness: { scope: :contact_form_id }
end

它确保没有重复 user / contact_form 创建行时进行配对。

我还在该表上设置了索引,以确保db级别的唯一性:

t.index ["user_id", "contact_form_id"], name: "index_contact_form_userships_on_user_id_and_contact_form_id", unique: true

test 'An error is raised if a user is added to a form more than once' do
  contact_form = ContactForm.create
  user = users(:user_1)

  assert_raises(ActiveRecord::RecordInvalid) do
    2.times do
      contact_form.users << user
    end
  end
end

如何在db级别测试唯一性?有没有办法 <<