simple_service_object.rb

#   # USAGE
#
#   # Include the concern in the class
#   class Settings::CreateSetting
#     include SimpleServiceObject
#
#     # optional: include validation
#     validates :shop, presence: true
#
#     # mandatory: initialize the command with keyword arguments
#     def initialize(shop)
#       @shop = shop
#     end
#
#     # mandatory: define a #call method.
#     # Return value is available through #payload
#     # Errors are available through #errors (uses ActiveModel for compatibility with view helpers, I18n etc.)
#     def call
#       create_setting
#     end
#
#     private
#
#     attr_reader :shop
#
#     def create_setting
#       languages = []
#       languages << shop.default_locale ||= "en"
#
#       Setting.create(shop: shop, languages: languages)
#     end
#   end
#
# #In your locale file you can setup custom errors
#
# #config/locales/en.yml
# en:
#   activemodel:
#     errors:
#       models:
#         settings
#           create_setting:
#             failure: "Could not create setting!"
#
#
# # In your Controller:
#
# class SettingsController < ApplicationController
#   def create
#     # initialize and execute the service
#     # NOTE: `.call` is a shortcut for `.new(args).call`
#     service = Settings::CreateSetting.call(current_shop)
#
#     # service.payload will contain the return value (even if invalid)
#
#     @setting = service.payload
#
#     # check service outcome
#     if service.success?
#       redirect_to @setting
#     else
#       flash.now[:alert] = t(service.errors.full_messages.to_sentence)
#       render :new
#       # in the view it will have access to @setting and can 
#       # show the validation errors normally @setting.errors...
#       # and generate forms: <%= form_with model: @setting do |form| %>
#     end
#   end
# end
#
# # TEST WITH RSPEC
#
# require "rails_helper"
#
# RSpec.describe Settings::CreateSetting do
#   describe ".call" do
#     it "creates setting with default language" do
#       shop = create(:shop)
#       expect(shop.setting).to be nil
#
#       service = Settings::CreateSetting.call(shop: shop)
#
#       expect(service.errors).to be_empty
#       expect(service.success?).to be true
#       expect(service.payload).to be_instance_of Setting
#
#       expect(shop.setting).not_to be nil
#       expect(shop.setting.languages).to eq(["en"])
#     end
#   end
# end
#
# # Inspired by https://github.com/nebulab/simple_command
#

module SimpleServiceObject
  extend ActiveSupport::Concern
  include ActiveModel::Validations

  included do
    attr_reader :payload
  end

  class_methods do
    def call(*args, **kwargs)
      new(*args, **kwargs)._call
    end
  end

  def _call
    fail NotImplementedError unless defined?(call)
    @called = true
    if valid?
      @payload = call
      # If an invalid ActiveRecord/ActiveModel record is returned by :call, add a default
      # :failure error, which also sets :success? to false. Allows us to return invalid
      # records for easier use in views and forms.
      errors.add(:base, :failure) if @payload&.errors&.any?
    end
    self
  end

  def success?
    called? && !failure?
  end
  alias successful? success?

  def failure?
    called? && errors.any?
  end

  # Provided by ActiveModel::Validations, fail loudly if removed.
  def errors
    fail NotImplementedError unless defined?(super)
    super
  end

  private

  def called?
    @called ||= false
  end
end