Mongo relationship issue

repro.rb

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'mongoid', '~> 7.1.4'
  gem 'minitest', '~> 5.14.1', require: ['minitest', 'minitest/autorun']
end

class Parent
  include Mongoid::Document

  belongs_to :child
end

class Child
  include Mongoid::Document

  has_one :parent

  def a_public_method
    'public'
  end

  private

  def a_private_method
    'private'
  end
end

class TestRelationships < Minitest::Test
  def test_child_private_methods_stay_private
    parent = Parent.new
    child = Child.new

    parent.child = child

    assert_raises NoMethodError do
      child.a_private_method
    end

    assert_raises NoMethodError do
      parent.child.a_private_method
    end
  end

  def test_child_methods_are_defined
    parent = Parent.new
    child = Child.new

    parent.child = child

    assert_equal 'method', defined?(child.a_public_method)
    assert_equal 'method', defined?(parent.child.a_public_method)
  end
end