Mongoid 7.0.2 regression in class_name detection

namespaced_models.rb

# frozen_string_literal: true

begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update your Bundler"
  raise e
end

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 "activemodel", "5.2.1"
  gem "openssl", "2.1.2"
  gem "mongoid", "7.0.2"
  # gem "mongoid", "7.0.1"
end

require "active_model"
require "mongoid"
require "minitest/autorun"
require "logger"

# Ensure backward compatibility with Minitest 4
Minitest::Test = MiniTest::Unit::TestCase unless defined?(Minitest::Test)

class Other
  include Mongoid::Document
end

class Space
  include Mongoid::Document
end

class Space::Child < Space
  belongs_to :other
  # belongs_to :other, class_name: 'Other' # workaround for 7.0.2
end

class BugTest < Minitest::Test
  def test_mongoid
    other = Other.new
    assert other.valid?

    child = Space::Child.new other: other
    assert child.valid?
  end
end