A bug I discovered with duplicating ActiveStorage records and the .attached? method. Rails issue: https://github.com/rails/rails/issues/40166

active_storage_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" }

  # Activate the gem you are reporting the issue against.
  gem "rails", "6.0.3"
  gem "sqlite3"
end

require "active_record/railtie"
require "active_storage/engine"
require "tmpdir"

class TestApp < Rails::Application
  config.root = __dir__
  config.hosts << "example.org"
  config.eager_load = false
  config.session_store :cookie_store, key: "cookie_store_key"
  secrets.secret_key_base = "secret_key_base"

  config.logger = Logger.new($stdout)
  Rails.logger  = config.logger

  config.active_storage.service = :local
  config.active_storage.service_configurations = {
      local: {
          root: Dir.tmpdir,
          service: "Disk"
      }
  }
end

ENV["DATABASE_URL"] = "sqlite3::memory:"

Rails.application.initialize!

require ActiveStorage::Engine.root.join("db/migrate/20170806125915_create_active_storage_tables.rb").to_s

ActiveRecord::Schema.define do
  CreateActiveStorageTables.new.change

  create_table :users, force: true
end

class User < ActiveRecord::Base
  has_one_attached :profile
end

require "minitest/autorun"

class BugTest < Minitest::Test
  def test_duplicate_without_checking_attached
    # create original user
    user = User.create!(
        profile: {
            content_type: "text/plain",
            filename: "dummy.txt",
            io: ::StringIO.new("dummy"),
        }
    )

    # duplicate user
    new_user = user.dup
    new_user.save!

    # duplicate attachment
    image_io = user.profile.download
    ct = user.profile.content_type
    fn = user.profile.filename
    new_blob = ActiveStorage::Blob.create_and_upload!(
      io: StringIO.new(image_io),
      filename: fn,
      content_type: ct,
    )

    # attach duplicated profile to new_user
    new_user.profile.attach(new_blob)

    # most recent attachment's record_id will match new_user.id
    assert_equal new_user.id, ActiveStorage::Attachment.last.record_id
  end

  def test_duplicate_checking_attached_first
    # create original user
    user = User.create!(
        profile: {
            content_type: "text/plain",
            filename: "dummy.txt",
            io: ::StringIO.new("dummy"),
        }
    )

    # check for attachments on user first <-- source of bug?
    user.profile.attached?

    # duplicate user
    new_user = user.dup
    new_user.save!

    # duplicate attachment
    image_io = user.profile.download
    ct = user.profile.content_type
    fn = user.profile.filename
    new_blob = ActiveStorage::Blob.create_and_upload!(
        io: StringIO.new(image_io),
        filename: fn,
        content_type: ct,
        )

    # attach duplicated profile to new_user
    new_user.profile.attach(new_blob)

    # FAILING TEST: most recent attachment's record_id will NOT match new_user.id
    assert_equal new_user.id, ActiveStorage::Attachment.last.record_id
  end
end