# app/mailers/user_mailer.rb
class UserMailer < ApplicationMailer
  default from: 'noreply@example.com'

  def welcome_email(user)
    @user = user

# Gemfile
gem 'view_component'

# app/components/button_component.rb
class ButtonComponent < ViewComponent::Base
  VARIANTS = %w[primary secondary danger].freeze

// app/javascript/controllers/dropdown_controller.js
import { Controller } from "@hotwired/stimulus"

export default class extends Controller {
  static targets = ["menu"]

# Generate engine
# rails plugin new billing --mountable

# lib/billing/engine.rb
module Billing
  class Engine < ::Rails::Engine

# Gemfile
gem 'graphql'

# Installation
# rails generate graphql:install

class Money
  include Comparable

  attr_reader :amount, :currency

  def initialize(amount, currency = 'USD')

# Installation
# rails active_storage:install
# rails db:migrate

# config/storage.yml
local:

# Gemfile
gem 'draper'

# app/decorators/user_decorator.rb
class UserDecorator < Draper::Decorator
  delegate_all

# Gemfile
group :development do
  gem 'rack-mini-profiler'
  gem 'memory_profiler'
  gem 'stackprof'  # For flamegraphs
  gem 'bullet'     # N+1 detection

# Basic middleware structure
class RequestTimerMiddleware
  def initialize(app)
    @app = app
  end

# app/policies/post_policy.rb
class PostPolicy < ApplicationPolicy
  class Scope < Scope
    def resolve
      if user.admin?
        scope.all

# app/channels/chat_channel.rb
class ChatChannel < ApplicationCable::Channel
  def subscribed
    # Subscribe to a specific room
    room = Room.find(params[:room_id])