https://github.com/activeadmin/activeadmin/issues/3647

aa_optional_belongs_to_bug_report.rb

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'

  gem 'rails', '4.2.7', require: false
  gem 'sqlite3', platform: :mri

  gem 'activerecord-jdbcsqlite3-adapter', platform: :jruby

  gem 'activeadmin', git: 'https://github.com/activeadmin/activeadmin', require: false

  gem 'inherited_resources', '~> 1.7', require: false
end

# prepare active_record database
require 'active_record'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

ActiveRecord::Schema.define do
  # Add your schema here
  create_table :countries, force: true do |t|
    t.string :name
    t.string :iso
  end

  create_table :cities, force: true do |t|
    t.integer :country_id, null: false
    t.string :name
  end

  create_table :users, force: true do |t|
    t.string :name
  end

  create_table :posts, force: true do |t|
    t.integer :user_id, null: false
    t.string :content
  end

end

# prepare rails app
require 'action_controller/railtie'
require 'action_view/railtie'
require 'active_admin'

class ApplicationController < ActionController::Base
end

class TestApp < Rails::Application
  config.root = File.dirname(__FILE__)
  config.logger = Logger.new(STDOUT)

  secrets.secret_token = 'secret_token'
  secrets.secret_key_base = 'secret_key_base'

  config.eager_load = false
end

# create models
class Country < ActiveRecord::Base
  has_many :regions, class_name: 'City'
end

class City < ActiveRecord::Base
  belongs_to :area, class_name: 'Country', foreign_key: :country_id
end

# configure active_admin
ActiveAdmin.setup do |config|
  config.authentication_method = false
  config.current_user_method = false
  config.comments = false
end

# initialize app
Rails.application.initialize!

# register pages and resources
ActiveAdmin.register_page 'Dashboard' do
  menu priority: 1, label: proc { I18n.t('active_admin.dashboard') }

  content do
    'Test Me'
  end
end

ActiveAdmin.register Country, as: 'Area' do
end

ActiveAdmin.register City, as: 'Region' do
  belongs_to :area, optional: true
  navigation_menu :default

  sidebar 'Area', if: proc { assigns[:area].present? } do
    attributes_table_for assigns[:area] do
      row(:id) { auto_link assigns[:area] }
      row :name
      row :iso
    end
  end

end

# draw active_admin routes
Rails.application.routes.draw do
  ActiveAdmin.routes(self)
end

# prepare tests
require 'minitest/autorun'
require 'rack/test'

class BugTest < Minitest::Test
  include Rack::Test::Methods

  def before_setup
    @country = Country.create! name: 'Wonderland', iso: 'WL'
    @city = City.create! area: @country, name: 'Wonder City'
  end

  def test_admin_areas_index
    get '/admin/areas'
    assert last_response.ok?
    assert_match 'Wonderland', last_response.body
  end

  def test_admin_area_show
    get "/admin/areas/#{@country.id}"
    assert last_response.ok?
    assert_match 'Wonderland', last_response.body
  end

  def test_admin_regions_index
    get '/admin/regions'
    assert last_response.ok?
    assert_match 'Wonder City', last_response.body
  end

  def test_admin_region_show
    get "/admin/regions/#{@city.id}"
    assert last_response.ok?
    assert_match 'Wonder City', last_response.body
  end

  def test_admin_area_regions_index
    get "/admin/areas/#{@country.id}/regions"
    assert last_response.ok?
    assert_match 'Wonder City', last_response.body
  end

  def test_admin_area_region_show
    get "/admin/areas/#{@country.id}/regions/#{@city.id}"
    assert last_response.ok?
    assert_match 'Wonder City', last_response.body
  end

  private

  def app
    Rails.application
  end
end