https://github.com/rails/rails/pull/26865

rails-uniqueness-validation-scope-with-polymorphic-association_active_record_master.rb

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"
  gem "rails", github: "rails/rails"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :events, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.integer :commentable_id
    t.string :commentable_type
    t.string :slug
  end
end

class Post < ActiveRecord::Base
  has_many :comments, as: :commentable
end

class Event < ActiveRecord::Base
  has_many :comments, as: :commentable
end

class Comment < ActiveRecord::Base
  belongs_to :commentable, polymorphic: true

  validates :slug, uniqueness: { scope: [:commentable] }
end

class BugTest < Minitest::Test
  def test_uniqueness_validation_scope_with_polymorphic_association
    post = Post.create!
    post.comments << Comment.create!

    event = Event.create!
    event.comments << Comment.create!

    assert_equal 1, post.comments.count
    assert_equal 1, event.comments.count
  end
end

I did not find any mention of the reported bug except this one mentioned on April 2010.

You can run this file using ruby rails-uniqueness-validation-scope-with-polymorphic-association_active_record_master.rb as suggested on http://guides.rubyonrails.org/contributing_to_ruby_on_rails.html#create-an-executable-test-case.