Validates presence for null objects broken in Active Record

active_record_test.rb

# frozen_string_literal: true

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"

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

  # Activate the gem you are reporting the issue against.
  gem "activerecord", "5.2.0"
  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 :posts, force: true do |t|
  end

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

class Post < ActiveRecord::Base
  has_many :comments
end

class NullPost
  def marked_for_destruction?
    false
  end

  def blank?
    true
  end
end

class Comment < ActiveRecord::Base
  belongs_to :post

  validates_presence_of :post

  def post
    super || NullPost.new
  end
end

class CommentWithActiveModelValidation < ActiveRecord::Base
  self.table_name = 'comments'

  belongs_to :post

  validates_with ActiveModel::Validations::PresenceValidator, attributes: [:post]

  def post
    super || NullPost.new
  end
end

class BugTest < Minitest::Test
  def test_active_record_presence_validator_handles_null_object
    comment = Comment.new
    comment.valid?

    assert comment.errors[:post].present?
  end

  def test_active_model_presence_validator_handles_null_object
    comment = CommentWithActiveModelValidation.new
    comment.valid?

    assert comment.errors[:post].present?
  end
end