bug with inclusion of polymorphic association

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

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.