class Classification < ApplicationRecord
  belongs_to :classifiable, polymorphic: true
  belongs_to :topic

  validates :classifiable_id, :uniqueness => { :scope=> [:topic_id, :classifiable_type] }
end

require "rails_helper"

describe Classification do
  it { should belong_to(:classifiable) }
  it { should belong_to(:topic) }

  it "ensures uniqueness" do
    create(:classification)
    should validate_uniqueness_of(:classifiable_id).scoped_to([:topic_id, :classifiable_type])
  end
end

Failures:

  1) Classification ensures uniqueness
     Failure/Error: should validate_uniqueness_of(:classifiable_id).scoped_to([:topic_id, :classifiable_type])
     
     ActiveRecord::SubclassNotFound:
       Invalid single-table inheritance type: Show is not a subclass of Shoulda::Matchers::ActiveRecord::Uniqueness::TestModels::Producu
     # ./spec/models/classification_spec.rb:9:in `block (2 levels) in <top (required)>'