bug with inclusion of polymorphic association

Run the embedded Minitest reproduction and compare test_patch_linkage_options_with_android with test_fetch_option_linked_with_android. Trace OptionResource's polymorphic optionable relationship through the JSONAPI controller and route. Done means both requests return successfully and the optionable association can be linked and included as the tests intend.

reproduced with gist

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', platform: :mri

  gem 'activerecord-jdbcsqlite3-adapter',
      git: 'https://github.com/jruby/activerecord-jdbc-adapter',
      branch: 'rails-5',
      platform: :jruby

  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 :options, force: true do |t|
    t.integer :optionable_id
    t.string :optionable_type
    t.boolean :enabled, default: false, null: false
  end

  create_table :androids, force: true do |t|
    t.string :version_name
  end
end

# create models
class Option < ActiveRecord::Base
  belongs_to :optionable, polymorphic: true, required: false
end

class Android < ActiveRecord::Base
  has_one :option, as: :optionable
end

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

class ApplicationController < ActionController::Base
end

# prepare jsonapi resources and controllers
class OptionsController < ApplicationController
  include JSONAPI::ActsAsResourceController
end

class OptionResource < JSONAPI::Resource
  attribute :enabled
  has_one :optionable, polymorphic: true, class_name: 'Android'
end

class AndroidResource < JSONAPI::Resource
  attribute :version_name
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
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 :options, only: [:show, :update]
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 test_patch_linkage_options_with_android
    android = Android.create! version_name: '1.0'
    option = Option.create!
    json_request = {
        data: {
            id: option.id.to_s,
            type: 'options',
            relationships: {
                optionable: {
                    data: {
                        id: android.id.to_s,
                        type: 'androids'
                    }
                }
            }
        }
    }
    patch "/options/#{option.id}?include=optionable", json_request.to_json, json_api_headers
    assert last_response.ok?
    assert android, option.reload.optionable
  end

  def test_fetch_option_linked_with_android
    android = Android.create! version_name: '1.0'
    option = Option.create! optionable: android
    get "/options/#{option.id}?include=optionable", nil, json_api_headers
    assert last_response.ok?
  end

  private

  def app
    Rails.application
  end
end