Polymorphic relations problem: undefined method for nil:NilClass

Reproducir el BugTest incluido, especialmente test_get_party_via_contact_medium, utilizando los recursos polimórficos ContactMedium, Individual y Organization. Comenzar con lib/jsonapi/relationship.rb:66 y lib/jsonapi/active_relation/join_manager.rb:93; se considera terminado cuando la solicitud GET /contact_media/:id/party devuelve correctamente el resultado esperado según la aserción.

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• Problem/bug report.
• Feature request.
• Request for support. Note: Please try to avoid submitting issues for support requests. Use Gitter instead.

Checklist before submitting:

• I've searched for an existing issue.
• I've asked my question on Gitter and have not received a satisfactory answer.
• I've included a complete bug report template. This step helps us and allows us to see the bug without trying to reproduce the problem from your description. It helps you because you will frequently detect if it's a problem specific to your project.
• The feature I'm asking for is compliant with the JSON:API spec.

Description

I seem to have bumped into a problem with polymorphic relations I'm unable to solve. Here's a description and a standalone bug report describing it. I'm on JR 0.10.2, Rails 6.0.2.1 and Ruby 2.6.5.

The model in question is composed of three ActiveRecord models: ContactMedium, Individuals and Organizations and the corresponding JR resources. The latter may have many contact media as party and one contact medium can belong to either kind of party.

The problem comes up when trying to get the individual or organisation as party via the contact media like this: GET /contact-media/1/party

The logic works on ActiveRecord level but not in the JR level.

Using the relationship results in the following error in the included example case:

Internal Server Error: undefined method `downcase' for nil:NilClass
.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/bundler/gems/jsonapi-resources-978f590f85fe/lib/jsonapi/relationship.rb:66:in 
`block (3 levels) in polymorphic_types'

The error is similar in the actual app but with a mild difference (that also included in the included pastebin link):

Internal Server Error: undefined method `left' for nil:NilClass
.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/jsonapi-resources-0.10.2/lib/jsonapi/active_relation/join_manager.rb:93:in 
`alias_from_arel_node'`

I'm suspecting this behaviour to be a bug. If I'm doing something wrong instead, please let me know. Workaround suggestions also welcome!

Bug reports:

Partial report in pastebin

https://pastebin.com/V0CfEs3F

Standalone bug report using the template

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