ActiveRecord polymorphic associations don't accept a custom primary key?

polymorphic_has_one_primary_key.rb

require 'bundler/inline'
require 'minitest/autorun'

gemfile true do
  source 'https://rubygems.org'
  gem 'activerecord', require: 'active_record'
  gem 'sqlite3'
end

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Schema.define do
  create_table :users, force: true do |t|
    t.integer :other_id
  end
  create_table :attachments, force: true do |t|
    t.string :record_type
    t.integer :record_id
  end
end

class User < ActiveRecord::Base
  has_one :attachment, as: :record, inverse_of: :record, primary_key: :other_id
end

class Attachment < ActiveRecord::Base
  belongs_to :record, polymorphic: true, touch: true
end

class PolymorphicHasOneTest < Minitest::Spec
  it 'uses the custom primary key' do
    user = User.create!(other_id: 42)
    attachment = user.create_attachment!

    # This assertion fails because `attachment.record_id` is actually `user.id`
    assert_equal 42, attachment.record_id
  end
end