Where should this code go?
│
├─ Is it data validation, associations, or simple business logic?
│   └─ → Model (rich models first!)
│
├─ Is it shared behavior across models?
│   └─ → Concern
│
├─ Is it business state tracking (who/when/why)?
│   └─ → State Record (see: state-records pattern)
│
├─ Does it orchestrate 3+ models or call external APIs?
│   └─ → Service Object (with Result pattern)
│
├─ Is it a complex database query (3+ joins, aggregations)?
│   └─ → Query Object
│
├─ Is it view/display formatting?
│   └─ → Presenter (SimpleDelegator)
│
├─ Is it authorization logic?
│   └─ → Pundit Policy
│
├─ Is it reusable UI with logic?
│   └─ → ViewComponent
│
├─ Is it async/background work?
│   └─ → Shallow Job (Solid Queue)
│
├─ Is it a complex form (multi-model, wizard)?
│   └─ → Form Object
│
├─ Is it a transactional email?
│   └─ → Mailer
│
└─ Is it HTTP request/response handling only?
    └─ → Controller (keep it thin!)

class Order < ApplicationRecord
  include Closeable  # State-as-records concern

  belongs_to :user
  has_many :line_items, dependent: :destroy

  validates :total_cents, presence: true, numericality: { greater_than: 0 }

  scope :recent, -> { order(created_at: :desc) }
  scope :pending, -> { where.missing(:closure) }

  def add_item(product, quantity: 1)
    line_items.create!(product: product, quantity: quantity, price_cents: product.price_cents)
    recalculate_total!
  end

  private

  def recalculate_total!
    update!(total_cents: line_items.sum("price_cents * quantity"))
  end
end

module Orders
  class CheckoutService
    def call(user:, cart:, payment_method_id:)
      order = nil

      ActiveRecord::Base.transaction do
        order = user.orders.create!(total_cents: cart.total_cents)
        cart.items.each { |item| order.add_item(item.product, quantity: item.quantity) }
        Inventory::ReserveService.new.call(order: order)
      end

      Payments::ChargeService.new.call(order: order, payment_method_id: payment_method_id)
      OrderMailer.confirmation(order).deliver_later
      Result.new(success: true, data: order)
    rescue ActiveRecord::RecordInvalid => e
      Result.new(success: false, error: e.message)
    end
  end
end

# GOOD: New resource for publishing
resources :posts do
  resource :publication, only: [:create, :destroy]
end
# POST /posts/:post_id/publication → Publications#create
# DELETE /posts/:post_id/publication → Publications#destroy

# BAD: Custom action
resources :posts do
  member do
    post :publish
    post :unpublish
  end
end

app/
├── channels/            # Action Cable channels
├── components/          # ViewComponents (UI + logic)
├── controllers/
│   └── concerns/        # Shared controller behavior
├── forms/               # Form objects
├── jobs/                # Background jobs (Solid Queue)
├── mailers/             # Action Mailer classes
├── models/
│   └── concerns/        # Shared model behavior
├── policies/            # Pundit authorization
├── presenters/          # View formatting
├── queries/             # Complex queries
├── services/            # Business logic (use sparingly)
│   └── result.rb        # Shared Result class
└── views/
    └── components/      # ViewComponent templates

# app/services/result.rb
class Result
  attr_reader :data, :error, :code

  def initialize(success:, data: nil, error: nil, code: nil)
    @success = success
    @data = data
    @error = error
    @code = code
  end

  def success? = @success
  def failure? = !@success

  def self.success(data = nil) = new(success: true, data: data)
  def self.failure(error, code: nil) = new(success: false, error: error, code: code)
end