poly_association_reference.rb

# Activate the gem you are reporting the issue against.
gem 'activerecord', '4.1.1'
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.references :commentable, polymorphic: true
  end
end

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

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

class BugTest < Minitest::Test
  def test_polymorphic_chained_association_reference
    post = Post.new
    post.comments.new
    comment = post.comments.first

    assert_equal post, comment.commentable
  end
end

# Non-polymorphic example

# ActiveRecord::Schema.define do
#   create_table :posts do |t|
#   end

#   create_table :comments do |t|
#     t.references :post
#   end
# end

# class Post < ActiveRecord::Base
#   has_many :comments
# end

# class Comment < ActiveRecord::Base
#   belongs_to :post
# end

# class BugTest < Minitest::Test
#   def test_non_polymorphic_chained_association_reference
#     post = Post.new
#     post.comments.new
#     comment = post.comments.first

#     assert_equal post, comment.post
#   end
# end