Show unscope behavior on subqueries

rails_unscope_subquery.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 "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.integer :num_comments
  end

  create_table :posts_relations, force: true do |t|
    t.integer :subject_id
    t.integer :object_id
  end

  create_table :comments, force: true do |t|
    t.integer :post_id
  end

  add_foreign_key :comments, :posts
  add_foreign_key :posts_relations, :posts, column: :subject_id
  add_foreign_key :posts_relations, :posts, column: :object_id
end

class Post < ActiveRecord::Base
  has_and_belongs_to_many :children, join_table: 'posts_relations', class_name: 'Post', association_foreign_key: :object_id, foreign_key: :subject_id
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

(2..4).each do |i|
  post = Post.create!(num_comments: i)
  i.times do
      Comment.create!(post: post)
  end
end

posts = Post.order(:id).all
posts[0].children = posts[1..-1]
posts[0].save

class BugTest < Minitest::Test
  def test_sanity
    assert_equal 3, Post.count
    assert_equal 9, Comment.count
    Post.find_each do |p|
      assert_equal p.num_comments, Comment.where(post: p).count
    end
  end

  def test_direct_unscope
    p1 = Post.first
    p2 = Post.last
    refute_equal p1, p2
    query = Comment.where(post: p1).unscope(where: :post_id).where(post: p2)
    assert_equal p2.num_comments, query.count
  end

  def test_subquery_unscope
    posts = Post.order(:id).all
    p2 = posts[0].children.where(id: 2)
    p3 = posts[0].children.where(id: 3)
    refute_equal p2, p3
    query = Comment.where(post: p2).unscope(where: :post_id).where(post: p3)
    assert_equal p3.first.num_comments, query.count
  end

  def test_avoid_subquery_unscope
    posts = Post.order(:id).all
    p2 = posts[0].children.where(id: 2)
    p3 = posts[0].children.where(id: 3)
    refute_equal p2, p3
    query = Comment.where(post: p2.first).unscope(where: :post_id).where(post: p3)
    assert_equal p3.first.num_comments, query.count
  end
end