wrap validation rules in a context

like_test.rb

require 'test_helper'

class LikeTest < ActiveSupport::TestCase
  should belong_to(:viewable)
  should belong_to(:user)

  context "validations" do
    subject { Like.create(user_id: 1, viewable_id: 2, liked: false) }

    should validate_uniqueness_of(:user_id).scoped_to(:viewable_id)
  end

end

ProTip: When testing a model's validation rules, you can wrap them inside a context to setup the expected objects / subject