STI with index broken on Rails 5

sti_with_index_broken_on_rails5.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 'arel', github: 'rails/arel'
  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 :resources, force: :cascade do |t|
    t.string   :type,       null: false, default: 'Resource'
    t.datetime :created_at, null: false
    t.datetime :updated_at, null: false
  end
end

class Resource < ActiveRecord::Base
end

class Channel < Resource
end

class BugTest < Minitest::Test
  def test_sti_with_index_on_type_raises_error_on_rails5_but_not_on4
    # on Rails 4:
    # => Everything Ok

    # on Rails 5.0.0.beta1 (and master@9864bce):
    # => ActiveRecord::SubclassNotFound: Invalid single-table inheritance type: Resource is not a subclass of Channel

    Channel.new
  end
end

For googles, the discussion is here: rails/rails#23285