issue-38811.rb

require "bundler/inline"

gemfile(true) do
  source "https://rubygems.org"

  git_source(:github) { |repo| "https://github.com/#{repo}.git" }

  gem "rails", "6.0.2.2"
  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|
  end

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

class Post < ActiveRecord::Base
  has_many :comments, -> { unscope(where: :locale) }
end

class Comment < ActiveRecord::Base
  belongs_to :post

  default_scope { where(locale: :en) }
end

class BugTest < Minitest::Test
  def test_association_stuff
    post = Post.create!
    post.comments << Comment.create!(locale: nil)
    Comment.create(locale: :en)

    assert_equal "SELECT \"posts\".* FROM \"posts\" INNER JOIN \"comments\" ON \"comments\".\"post_id\" = \"posts\".\"id\"", Post.joins(:comments).to_sql
  end
end