Rails master fails when polymorphic associations have touch: true (as of a489bfe48bc0fbbb50a472bc66bd42d408c73fcb)

polymorphic-touch-bug.rb

unless File.exists?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    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, touch: true
end

class BugTest < MiniTest::Unit::TestCase
  def test_association_stuff
    # don't need to do anything
  end
end

See issue here

Removed :counter_cache => true, as it wasn't needed to reproduce