Polymorphic has_many/has_one with custom columns.

polymorphic_has_many.rb

unless File.exists? 'Gemfile'
  File.write('Gemfile', <<-GEMFILE)
source 'https://rubygems.org'
 gem 'rails' # Failing
 # gem 'rails', :git => 'https://github.com/ulissesalmeida/rails.git', :branch => 'polymorphic-has-one-many-custom-foreign-type'
gem 'sqlite3'
GEMFILE

system 'bundle --quiet'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

# 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 :commentable_id
    t.string :commentable_class
  end
end

class Post < ActiveRecord::Base
  has_many :comments, as: :commentable, foreign_type: 'commentable_class'
  has_one :comment, as: :commentable, foreign_type: 'commentable_class'
end

class Comment < ActiveRecord::Base
  belongs_to :commentable, polymorphic: true, foreign_type: 'commentable_class'
end

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

    assert_equal 1, post.comments.count
    assert_equal 1, Comment.count
    assert_equal post.id, Comment.first.commentable.id
    assert_equal post.comment.id, Comment.first.id
  end
end