Reproduction script: Wrong class using reflection on polymorphic belongs_to association

gistfile1.rb

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'sqlite3'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

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 do |t|
  end
  create_table :comments do |t|
    t.integer :parent_id
    t.string  :parent_type
  end
end

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

class Comment < ActiveRecord::Base
  belongs_to :parent, :polymorphic => true
end

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

    assert_equal Post, Comment.reflect_on_all_associations(:belongs_to).first.klass
    assert_equal Post, post.comments.first.association(:parent).reflection.klass
  end
end