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False positive for enum without DB column in rails 7.2 in parallelized tests

While updating an app to 7.2, my CI started throwing a lot of errors around enums:

RuntimeError: Undeclared attribute type for enum 'post_type' in Post. Enums must be backed by a database column or declared with an explicit type via `attribute`.

These are false positives as (1) the DB column exists (2) the tests worked locally/work when not executed in parallel.

I think the actual issue might not be related to enums, but just be some underlying problem in how attributes are initialized. However, I'm not knowledgeable enough about these rails internals to be really sure of what is going on

Steps to reproduce

  • Add an enum backed by a Postgres enum to a model
  • Add a test that calls the model in some way that loads the attribute info
  • Run tests in parallel
  • Run tests using config.eager_load = true (This is needed in my actual app but not in the reproduction below)

I had to add a bunch of stuff to the basic script to reproduce the error and had to create a separate schema file. There might be a better way to reproduce this

# frozen_string_literal: true

ENV['RAILS_ENV'] ||= 'test'

require "bundler/inline"

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

  gem "rails", "~> 7.2.0"
  # If you want to test against edge Rails replace the previous line with this:
  # gem "rails", github: "rails/rails", branch: "main"

  gem "pg", "~> 1.5.7"

  gem "minitest", "5.24.1" # https://github.com/minitest/minitest/issues/1007
end

require "active_record"
require "action_controller/railtie"
require "logger"

ActiveRecord::Base.configurations = { 
  "test" => {
    "adapter"=>"postgresql",
    "database"=>"test",
    "host"=>"localhost",
    "username"=>"rails",
    "password"=>"rails"
  }
}

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(
  adapter:  "postgresql",
  host:     "localhost",
  username: "rails",
  password: "rails",
  database: "test")
ActiveRecord::Base.logger = Logger.new(STDOUT)


class Application < Rails::Application
  # Setting eager load to `true` matters in my test setup
  # but this does not make a difference in the reproduction
  config.eager_load = false
end

Rails.application.initialize!

class Post < ActiveRecord::Base
  enum :post_type, { one: 'one', two: 'two', three: 'three' }
end


class ActiveSupport::TestCase
  parallelize(
    workers: 0, # The amount of workers doesn't matter, as long as `parallelize` is used
    threshold: 1 # The threshold does matter, if you set this to 2 the tests will work
  )
end

require "rails/test_help"

class BugTest < ActiveSupport::TestCase
  def test_one
    Post.new

    assert true
  end

  def test_two
    Post.new

    assert true
  end
end

In a separate db/schema.rb file

ActiveRecord::Schema.define do
  create_enum "my_enum", ["one", "two", "three"]

  create_table :posts, force: true do |t|
    t.enum :post_type, enum_type: :my_enum
  end
end

Expected behavior

The DB column should be detected, since this worked in rails 7.1.X (and many versions before that)

Actual behavior

An error is throw saying that the enum does not have a matching DB column

RuntimeError: Undeclared attribute type for enum 'post_type' in Post. Enums must be backed by a database column or declared with an explicit type via `attribute`.
    vendor/ruby/gems/activerecord-7.2.0/lib/active_record/enum.rb:259:in `block in _enum'
    vendor/ruby/gems/activemodel-7.2.0/lib/active_model/attribute_registration.rb:71:in `block in apply_to'
    vendor/ruby/gems/activemodel-7.2.0/lib/active_model/attribute_registration.rb:69:in `each'
    vendor/ruby/gems/activemodel-7.2.0/lib/active_model/attribute_registration.rb:69:in `apply_to'
    vendor/ruby/gems/activemodel-7.2.0/lib/active_model/attribute_registration.rb:87:in `block in apply_pending_attribute_modifications'
    vendor/ruby/gems/activemodel-7.2.0/lib/active_model/attribute_registration.rb:86:in `each'
    vendor/ruby/gems/activemodel-7.2.0/lib/active_model/attribute_registration.rb:86:in `apply_pending_attribute_modifications'
    vendor/ruby/gems/activerecord-7.2.0/lib/active_record/attributes.rb:249:in `_default_attributes'
    vendor/ruby/gems/activemodel-7.2.0/lib/active_model/attribute_registration.rb:38:in `attribute_types'
    vendor/ruby/gems/activerecord-7.2.0/lib/active_record/attribute_methods.rb:260:in `_has_attribute?'
    vendor/ruby/gems/activerecord-7.2.0/lib/active_record/inheritance.rb:61:in `new'
    script.rb:67:in `test_one'

System configuration

Rails version: 7.2

Ruby version: 3.2.4