Rails 4.2 with Ruby 2.4.9: undefined method `root' for JSONAPI::Rails:Module

base_url が組み立てられている lib/jsonapi/acts_as_resource_controller.rb:155 から開始し、そこで使用されている Rails アプリケーション参照を確認します。config/routes.rb の Api::TestsController の例を通じてリクエストを再現し、その結果、undefined-method エラーが発生しなくなり、期待される base URL が引き続き構築されることを確認します。

This issue is a (choose one):

• 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

As the title: controller raises the exception:

E, [2020-10-29T02:29:40.427579 #92859] ERROR -- : Internal Server Error: undefined method `application' for JSONAPI::Rails:Module /*/.rbenv/versions/2.4.9/lib/ruby/gems/2.4.0/gems/jsonapi-resources-0.10.2/lib/jsonapi/acts_as_resource_controller.rb:155:in `base_url'

which can be fixed with

    def base_url
-      @base_url ||= "#{request.protocol}#{request.host_with_port}#{Rails.application.config.relative_url_root}"
+      @base_url ||= "#{request.protocol}#{request.host_with_port}#{::Rails.application.config.relative_url_root}"
    end

# config/routes.rb
  namespace :api do
    jsonapi_resources :tests, only: %i[index]
  end

# app/controllers/api/tests_controller.rb
module Api
  class TestsController < ::Api::BaseController
  end
end