STI + enum fail in Rails 5.0.0.beta1

failing_test_enum_sti.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', '>= 5.0.0.beta1', '< 5.1'
# gem 'rails'  ## use this instead to test against the current Rails (4.2)
  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 :activities, force: true  do |t|
    t.string :type
    t.integer :status, default: 0,  null: false
  end
end

class Activity < ActiveRecord::Base
  enum status: %i(pending failed successful)
end

class Snapshot < Activity
end

class Adjustment < Activity
end

class BugTest < Minitest::Test
  def test_sti_enum
    Adjustment.create! status: 'successful'
    Snapshot.create!   status: 'successful'

    # The following passes both on Rails 4.2 and on Rails 5.0.0.beta1.
    assert Snapshot.first.is_a?(Snapshot)

    # The following fails only on Rails 5.0.0.beta1 (returns an Adjustment).
    assert Snapshot.successful.first.is_a?(Snapshot)
  end
end

When run against Rails 4.2, these are the SQL queries I see in the log file:

# assert Snapshot.first.is_a?(Snapshot)
SELECT  "activities".* FROM "activities" WHERE "activities"."type" IN ('Snapshot')  ORDER BY "activities"."id" ASC LIMIT 1

# assert Snapshot.successful.first.is_a?(Snapshot)
SELECT  "activities".* FROM "activities" WHERE "activities"."type" IN ('Snapshot') AND "activities"."status" = ?  ORDER BY "activities"."id" ASC LIMIT 1  [["status", 2]]

However, when run against Rails 5.0.0.beta1, these are the SQL queries I see in the log file:

# assert Snapshot.first.is_a?(Snapshot)
SELECT  "activities".* FROM "activities" WHERE "activities"."type" IN ('Snapshot') ORDER BY "activities"."id" ASC LIMIT ?  [["LIMIT", 1]]

# assert Snapshot.successful.first.is_a?(Snapshot)
SELECT  "activities".* FROM "activities" WHERE "activities"."status" = ? ORDER BY "activities"."id" ASC LIMIT ?  [["status", 2], ["LIMIT", 1]]