Rails polymorphic issue

bug.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"
  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 :resource_id
    t.string :resource_type
  end
end

class Post < ActiveRecord::Base
  has_many :comments, as: :resource
end

class Comment < ActiveRecord::Base
  belongs_to :resource, polymorphic: true, optional: true
end

class BugTest < Minitest::Test
  def test_finding_comments_with_resource
    post = Post.create!
    post.comments << Comment.create!

    assert_equal 1, Comment.where(resource: post).size
  end

  def test_finding_comments_without_resource
    post = Post.create!
    post.comments << Comment.create!

    Comment.create!

    assert_equal 1, Comment.where(resource: nil).size
  end
end