Polymorphic relations problem: undefined method for nil:NilClass #1305

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

gemfile(true, ui: ENV['SILENT'] ? Bundler::UI::Silent.new : Bundler::UI::Shell.new) do
  source 'https://rubygems.org'

  gem 'rails', require: false
  gem 'sqlite3'
  gem 'i18n', '= 1.7.0'

  if ENV['JSONAPI_RESOURCES_PATH']
    gem 'jsonapi-resources', path: ENV['JSONAPI_RESOURCES_PATH'], require: false
  else
    gem 'jsonapi-resources', git: 'https://github.com/cerebris/jsonapi-resources', require: false
  end

end

# prepare active_record database
require 'active_record'

class NullLogger < Logger
  def initialize(*_args)
  end

  def add(*_args, &_block)
  end
end

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = ENV['SILENT'] ? NullLogger.new : Logger.new(STDOUT)
ActiveRecord::Migration.verbose = !ENV['SILENT']

ActiveRecord::Schema.define do
  # Add your schema here
  create_table :contact_media do |t|
    t.string :name
    t.references :party, polymorphic: true, index: true
  end

  create_table :individuals do |t|
    t.string :name
  end

  create_table :organizations do |t|
    t.string :name
  end
end

# create models
 class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true
end

class ContactMedium < ApplicationRecord
  belongs_to :party, polymorphic: true, inverse_of: :contact_media
end

class Individual < ApplicationRecord
  has_many :contact_media, as: :party
end

class Organization < ApplicationRecord
  has_many :contact_media, as: :party
end

# prepare rails app
require 'action_controller/railtie'
# require 'action_view/railtie'
require 'jsonapi-resources'

class ApplicationController < ActionController::Base
  protect_from_forgery with: :null_session
end

# prepare jsonapi resources and controllers
#

class IndividualsController < ApplicationController
  include JSONAPI::ActsAsResourceController
end

class OrganizationsController < ApplicationController
  include JSONAPI::ActsAsResourceController
end

class ContactMediaController < ApplicationController
  include JSONAPI::ActsAsResourceController
end

class PartiesController < ApplicationController
  include JSONAPI::ActsAsResourceController
end

class ContactMediumResource < JSONAPI::Resource
  attribute :name
  has_one :party, polymorphic: true
end

class IndividualResource < JSONAPI::Resource
  attribute :name
  has_many :contact_media
end

class OrganizationResource < JSONAPI::Resource
  attribute :name
  has_many :contact_media
end

class PartyResource < JSONAPI::Resource
end

class TestApp < Rails::Application
  config.root = File.dirname(__FILE__)
  config.logger = ENV['SILENT'] ? NullLogger.new : Logger.new(STDOUT)
  Rails.logger = config.logger

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

  config.eager_load = false
  config.hosts << "example.org"
end

# initialize app
Rails.application.initialize!

JSONAPI.configure do |config|
  config.json_key_format = :underscored_key
  config.route_format = :underscored_key
end

# draw routes
Rails.application.routes.draw do
  jsonapi_resources :contact_media do
    jsonapi_relationships
  end
  jsonapi_resources :individuals do
    jsonapi_relationships
  end
  jsonapi_resources :organizations do
    jsonapi_relationships
  end
end

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

# Replace this with the code necessary to make your test fail.
class BugTest < Minitest::Test
  include Rack::Test::Methods

  def json_api_headers
    {'Accept' => JSONAPI::MEDIA_TYPE, 'CONTENT_TYPE' => JSONAPI::MEDIA_TYPE}
  end

  def teardown
    Individual.delete_all
    ContactMedium.delete_all
  end

  def test_find_party_via_contact_medium
    individual = Individual.create(name: 'test')
    contact_medium = ContactMedium.create(party: individual, name: 'test contact medium')
    fetched_party = contact_medium.party
    assert_same individual, fetched_party, "Expect an individual to have been found via contact medium model's relationship 'party'"
  end

  def test_get_individual
    individual = Individual.create(name: 'test')
    ContactMedium.create(party: individual, name: 'test contact medium')
    get "/individuals/#{individual.id}"
    assert last_response.ok?
  end

  def test_get_party_via_contact_medium
    individual = Individual.create(name: 'test')
    contact_medium = ContactMedium.create(party: individual, name: 'test contact medium')
    get "/contact_media/#{contact_medium.id}/party"
    assert last_response.ok?, "Expect an individual to have been found via contact medium resource's relationship 'party'"
  end

  private

  def app
    Rails.application
  end
end