Issue found investigating rails/rails #21466

test_association.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'
  # NOTE: test passes on 4.1.13, fails against 4.2.0+
  # gem 'activerecord', '4.1.13'
  gem 'activerecord', '4.2.3'
  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 :users, force: true do |t|
    t.integer :customer_id
    t.string :name
    t.string :role
  end

  create_table :customers, force: true do |t|
    t.string :name
  end

  create_table :programs, force: true do |t|
    t.integer :customer_id
    t.string :name
  end
end

class User < ActiveRecord::Base
  belongs_to :owned_customer, class_name: "Customer", inverse_of: :owner, foreign_key: :customer_id
  has_many :owned_programs, inverse_of: :owner, through: :owned_customer, source: :programs
end

class Customer < ActiveRecord::Base
  has_one :owner, -> { where(:role => 'owner') }, class_name: "User", inverse_of: :owned_customer, dependent: :destroy
  has_many :programs, inverse_of: :customer, dependent: :restrict_with_exception
end

class Program < ActiveRecord::Base
  belongs_to :customer, inverse_of: :programs
  # Bug is here. Removing inverse_of makes the test pass
  # With inverse_of on AR 4.2.3, ruby 2.1.2p95 (2014-05-08 revision 45877) [x86_64-darwin13.0]:
  #
  #   1) Error:
  # BugTest#test_association_stuff:
  # NameError: uninitialized constant Program::Owner
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/inheritance.rb:158:in `compute_type'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/reflection.rb:271:in `compute_class'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/reflection.rb:267:in `klass'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/reflection.rb:393:in `inverse_of'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/reflection.rb:336:in `check_validity_of_inverse!'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/reflection.rb:877:in `check_validity_of_inverse!'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/reflection.rb:855:in `check_validity!'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/associations/association.rb:25:in `initialize'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/associations.rb:162:in `new'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/associations.rb:162:in `association'
  #   /opt/rubies/2.1.2/lib/ruby/gems/2.1.0/gems/activerecord-4.2.3/lib/active_record/associations/builder/association.rb:115:in `owner'
  #   test_associations.rb:88:in `test_association_stuff'
  has_one :owner, through: :customer, inverse_of: :owned_programs
end

class BugTest < Minitest::Test
  def test_association_stuff
    customer = Customer.create!(name: 'Nobody')
    program = Program.create!(name: 'Ed Norton Utilities', customer: customer)
    user = User.create!(name: 'Gorbypuff', role: 'owner', owned_customer: customer)

    new_program = Program.last
    assert new_program.owner.present?
    assert new_program.save
  end
end