Rails 4.2.4 + Paranoia issue for with_deleted on belongs_to

gistfile1.txt

# I've been troubleshooting an issue that cropped up in our app when we upgraded from
# rails 4.1 to 4.2. We make heavy use of the 'paranoia' gem to allow for soft-deleting
# records via a deleted_at attribute. The gem uses default scopes that exclude
# `deleted_at: nil` records from default queries.
# 
# In our join tables, we unscope the belongs_to relation because once the app has found
# its way to a record in the join table, it expects to be able to see the joined
# records, even if those records have been deleted.
# 
# Starting with Rails 4.2.4, it appears that this unscope on the belongs_to unexpectedly
# affects the has_many relation from the other side, unscoping that relation and
# causing records with deleted_at set to be returned.
# 
# I've created this test script to demonstrate the problem. The key parts of the
# example are the following:
# 
# ```
# class Product < ActiveRecord::Base
#   has_many :category_products
#   has_many :categories, through: :category_products
# 
#   default_scope { where deleted_at: nil }
# end
# 
# class CategoryProduct < ActiveRecord::Base
#   belongs_to :product, -> { unscope(where: :deleted_at) }
#   belongs_to :category, -> { unscope(where: :deleted_at) }
# end
# 
# class Category < ActiveRecord::Base
#   has_many :category_products
#   has_many :products, through: :category_products
# 
#   default_scope { where deleted_at: nil }
# end
# ```
# 
# With this setup, after marking a product's category as deleted, I expect
# that product.categories should no longer return the category. In 4.2.3 it did not,
# but in 4.2.4 and later, it does.
# 

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'
  # Activate the gem you are reporting the issue against.
  gem 'activerecord', '4.2.7'
  gem 'sqlite3'
end

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 :products, force: true do |t|
    t.string :name
    t.datetime :deleted_at
  end

  create_table :category_products, force: true do |t|
    t.integer :product_id
    t.integer :category_id
  end

  create_table :categories, force: true do |t|
    t.string :name
    t.datetime :deleted_at
  end

  create_table :category_product_visits, force: true do |t|
    t.string :user_name
    t.integer :category_product_id
  end
end

class Product < ActiveRecord::Base
  has_many :category_products
  has_many :categories, through: :category_products

  default_scope { where deleted_at: nil }
end

class CategoryProduct < ActiveRecord::Base
  # we unscope the belongs_to relations because if we are looking at a
  # CategoryProduct record (found via the CategoryProductVisit table, perhaps),
  # we expect to be able to see its category or product, even if the category or product
  # has been marked as deleted
  belongs_to :product, -> { unscope(where: :deleted_at) }
  belongs_to :category, -> { unscope(where: :deleted_at) }
end

class Category < ActiveRecord::Base
  has_many :category_products
  has_many :products, through: :category_products

  default_scope { where deleted_at: nil }
end

class CategoryProductVisit < ActiveRecord::Base
  belongs_to :category_product
end

class BugTest < Minitest::Test
  def test_association_stuff
    product1 = Product.create! name: 'Product 1'

    category1 = Category.create! name: 'Category 1'

    category1.products << [product1]

    product1.reload
    assert_equal 1, product1.categories.count

    # a user visits a product in a category. let's record that!
    visit1 = CategoryProductVisit.create! user_name: 'tom',
                                          category_product: category1.category_products.first


    # now we delete the category
    category1.deleted_at = DateTime.now
    category1.save!

    # the visit record can still see the product and category
    visit1.reload
    assert_equal visit1.category_product.product.name, 'Product 1'
    assert_equal visit1.category_product.category.name, 'Category 1'

    product1.reload
    # in rails 4.2.3, the following test passes, which is the desired behavior
    assert_equal 0, product1.categories.count

    # in rails 4.2.4 and later, product1.categories.count == 1, which is not desired behavior
    #assert_equal 1, product1.categories.count
  end
end