Problems joining through polymorphic association with active record

poly_test.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', github: 'rails/rails'
  gem 'arel', github: 'rails/arel'
  gem 'rack', github: 'rack/rack'
  gem 'sprockets', github: 'rails/sprockets'
  gem 'sprockets-rails', github: 'rails/sprockets-rails'
  gem 'sass-rails', github: 'rails/sass-rails'
  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 :posts, force: true  do |t|
    t.string :title
  end

  create_table :post_artifacts, force: true do |t|
    t.references :post
    t.integer :rule
    t.references :artifact, polymorphic: true
  end

  create_table :comments, force: true  do |t|
    t.text :content
  end
end

class Post < ActiveRecord::Base
  has_many :post_artifacts
  has_many :comments, through: :post_artifacts, source: :artifact, source_type: 'Comment'
  has_many :a_comments, -> { PostArtifact.a }, through: :post_artifacts, source: :artifact, source_type: 'Comment'
end

class Comment < ActiveRecord::Base
  has_many :post_artifacts, as: :artifact
  has_many :a_posts, -> { PostArtifact.a }, through: :post_artifacts, source: :post
end

class PostArtifact < ActiveRecord::Base
  enum rule: [:a, :b]
  belongs_to :post
  belongs_to :artifact, polymorphic: true
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    10.times {
      post.a_comments << Comment.create!
    }

    assert_equal 10, Post.includes(:comments).flat_map(&:comments).count #working
    assert_equal 10, Post.includes(:a_comments).flat_map(&:a_comments).count
  end
end