belongs_to_optional_true_with_invalid_foreign_key.rb

# frozen_string_literal: true

require "bundler/inline"

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

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

  gem "rails", github: "rails/rails", branch: "main"
  gem "sqlite3"
end

require "active_record"
require "minitest/autorun"
require "logger"

ActiveRecord::Base.establish_connection(adapter: "sqlite3", database: ":memory:")
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Base.belongs_to_required_by_default = true

ActiveRecord::Schema.define do
  create_table :posts, force: true do |t|
  end

  create_table :comments, force: true do |t|
    t.references :post, null: true
  end

  create_table :views, force: true do |t|
    t.references :post
  end
end

class Post < ActiveRecord::Base
end

class Comment < ActiveRecord::Base
  belongs_to :post, optional: true
end

class View < ActiveRecord::Base
  belongs_to :post
end

class BugTest < Minitest::Test
  def test_optional_true_with_existing_post_id
    post = Post.create!
    Comment.create!(post_id: post.id)
  end

  def test_optional_true_with_no_post_id
    Comment.create!
  end

  def test_optional_true_with_invalid_post_id
    assert_raises ActiveRecord::RecordInvalid do
      Comment.create!(post_id: 7685453412456)
    end
  end

  def test_with_existing_post_id
    post = Post.create!
    View.create!(post_id: post.id)
  end

  def test_with_no_post_id
    assert_raises ActiveRecord::RecordInvalid do
      View.create!
    end
  end

  def test_with_invalid_post_id
    assert_raises ActiveRecord::RecordInvalid do
      View.create!(post_id: 7685453412456)
    end
  end
end

I think somewhere ActiveRecord::Base.belongs_to_required_by_default = true needs to be set for this example to be correct. Otherwise Comment.create! would not raise an error, even if belongs_to :post would require the post to be present.

Right, thanks @Flixt