Issue: https://github.com/rails/rails/issues/13701

ar-pg-default-scope-join-test.rb

@db = ''
@user = ''
@password = ''

unless File.exist?('Gemfile')
  File.write('Gemfile', <<-GEMFILE)
    source 'https://rubygems.org'
    gem 'rails', github: 'rails/rails'
    gem 'arel', github: 'rails/arel'
    gem 'pg'
  GEMFILE

  system 'bundle'
end

require 'bundler'
Bundler.setup(:default)

require 'active_record'
require 'minitest/autorun'
require 'logger'
require 'pg'

conn = PG.connect
conn.exec("DROP DATABASE IF EXISTS #{@db}")
conn.exec("CREATE DATABASE #{@db}")
conn.close

ActiveRecord::Base.establish_connection(adapter: 'postgresql', database: @db, user: @user, password: @password )
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  create_table :articles do |t|
  end

  create_table :authors do |t|
  end

  create_join_table :articles, :authors, table_name: :authorships
end

class Article < ActiveRecord::Base
  has_many :authorships
  has_many :authors, through: :authorships
end

class Author < ActiveRecord::Base
  has_many :authorships
  has_many :articles, through: :authorships
end

class Authorship < ActiveRecord::Base
  belongs_to :author
  belongs_to :article
  default_scope -> { where(primary: true) }  # !!!
end

puts (Author.first || Author.create).articles.to_sql