module Api
  module V1
    class UsersController < ApplicationController
      before_action :authenticate_user!, except: [:index, :show]
      before_action :set_user, only: [:show, :update, :destroy]

<%# Fragment caching - caches rendered HTML %>
<% cache @product do %>
  <h1><%= @product.name %></h1>
  <p><%= @product.description %></p>
  <p>Price: $<%= @product.price %></p>
<% end %>

# Map - transform elements
[1, 2, 3, 4].map { |n| n * 2 }          # => [2, 4, 6, 8]
['alice', 'bob'].map(&:upcase)          # => ['ALICE', 'BOB']
users.map(&:email)                       # => ['alice@example.com', ...]

# Select/Reject - filter elements

# app/validators/order_validator.rb
class OrderValidator < ActiveModel::Validator
  def validate(record)
    validate_order_total(record)
    validate_items_availability(record)
    validate_shipping_address(record)

# Simple mixin
module Loggable
  def log(message)
    puts "[#{Time.now}] #{self.class.name}: #{message}"
  end
end

# BAD: N+1 query problem
@users = User.all
@users.each do |user|
  puts user.posts.count  # Fires query for each user!
end

class Timer
  def self.measure
    start_time = Time.now
    yield if block_given?
    end_time = Time.now
    end_time - start_time

class UserNotificationJob < ApplicationJob
  queue_as :default

  # Retry up to 5 times with exponential backoff
  retry_on StandardError, wait: :exponentially_longer, attempts: 5

require 'rails_helper'

RSpec.describe User, type: :model do
  subject(:user) { build(:user) }

  describe 'validations' do

class UserRegistrationService
  class Result
    attr_reader :user, :errors

    def initialize(success:, user: nil, errors: [])
      @success = success

class DynamicFinder
  def initialize(records)
    @records = records
  end

  def method_missing(method_name, *args, &block)

apiVersion: apps/v1
kind: Deployment
metadata:
  name: user-service
  labels:
    app: user-service