Rails Patterns Skill

Authority: FINAL

Purpose

Master Ruby on Rails development patterns, architecture, and best practices with deep expertise in Rails Engines for modular application development.

Core Principles

Convention Over Configuration - Follow Rails conventions for consistency
DRY (Don't Repeat Yourself) - Extract common patterns into reusable components
Fat Models, Skinny Controllers - Business logic in models, controllers coordinate
Service Objects for Complex Logic - Extract multi-model operations
Modularity with Engines - Build maintainable, isolated components

Rails Engines (CRITICAL SECTION)

Rails Engines are mini-applications that provide functionality to their host applications. They are the foundation for building modular, maintainable Rails applications and for creating reusable gems like Devise, Spree, and others.

Engine Types

Mountable Engine

A mountable engine is isolated from the host application with its own namespace. It acts like a completely separate application.

Creation:

rails plugin new my_engine --mountable

Characteristics:

Isolated namespace with isolate_namespace
Own routes mounted at a specific path
Own models, controllers, views in namespaced directories
Database tables prefixed with engine name
Independent from parent app (minimal coupling)

Directory Structure:

my_engine/
├── app/
│   ├── controllers/
│   │   └── my_engine/
│   │       ├── application_controller.rb
│   │       └── posts_controller.rb
│   ├── models/
│   │   └── my_engine/
│   │       └── post.rb
│   └── views/
│       └── my_engine/
│           └── posts/
├── config/
│   └── routes.rb
├── db/
│   └── migrate/
├── lib/
│   ├── my_engine/
│   │   ├── engine.rb
│   │   └── version.rb
│   └── my_engine.rb
├── test/
└── my_engine.gemspec

When to Use:

Building reusable functionality for multiple applications
Creating admin panels or CMS systems
Isolating large features for team organization
Planning to extract functionality into a gem
Need complete isolation from parent app

Full Engine

A full engine shares the namespace with the host application, providing less isolation.

Creation:

rails plugin new my_engine --full

Characteristics:

Shares namespace with parent app
Models/controllers not automatically namespaced
More tightly coupled to parent app
Easier access to parent app functionality
Simpler for internal modularization

Directory Structure:

my_engine/
├── app/
│   ├── controllers/
│   │   └── posts_controller.rb  # No namespace
│   ├── models/
│   │   └── post.rb  # No namespace
│   └── views/
│       └── posts/
├── config/
│   └── routes.rb
├── lib/
│   ├── my_engine/
│   │   └── engine.rb
│   └── my_engine.rb
└── my_engine.gemspec

When to Use:

Internal modularization only
Need easy access to parent app classes
Don't need strict isolation
Won't distribute as a gem
Prototyping engine functionality

Comparison Table

Feature	Mountable Engine	Full Engine
Namespace Isolation	Yes (`isolate_namespace`)	No
Route Mounting	Required	Optional
Table Prefixes	Yes (`my_engine_posts`)	No (`posts`)
Distribution as Gem	Ideal	Possible but not ideal
Access to Parent App	Indirect (configuration)	Direct
Complexity	Higher	Lower
Reusability	High	Medium

Engine Structure and Organization

Engine Class Definition

lib/my_engine/engine.rb:

module MyEngine
  class Engine < ::Rails::Engine
    isolate_namespace MyEngine

    # Engine configuration
    config.generators do |g|
      g.test_framework :rspec
      g.fixture_replacement :factory_bot
      g.factory_bot dir: 'spec/factories'
    end

    # Load decorators from parent app
    config.to_prepare do
      Dir.glob(Engine.root.join("app", "decorators", "**", "*_decorator*.rb")).each do |c|
        Rails.configuration.cache_classes ? require(c) : load(c)
      end
    end

    # Precompile engine assets
    initializer "my_engine.assets.precompile" do |app|
      app.config.assets.precompile += %w( my_engine/application.js my_engine/application.css )
    end

    # Add engine paths to parent app
    initializer "my_engine.add_middleware" do |app|
      app.middleware.use MyEngine::Middleware::CustomMiddleware
    end
  end
end

Namespace Isolation

What isolate_namespace does:

module MyEngine
  class Engine < ::Rails::Engine
    isolate_namespace MyEngine
  end
end

Effects:

Routes: All routes are namespaced under MyEngine
Tables: Database tables prefixed with my_engine_
Controllers: Inherit from MyEngine::ApplicationController
Models: Contained in MyEngine module
Helpers: Namespaced under MyEngine
URL Helpers: Prefixed with engine name

Engine Configuration

config/initializers/my_engine.rb in parent app:

MyEngine.configure do |config|
  config.default_locale = :en
  config.max_items_per_page = 25
  config.enable_caching = Rails.env.production?

  # Provide parent app models/controllers
  config.user_class = "User"
  config.admin_class = "Admin"

  # Callbacks
  config.after_create_post = ->(post) { NotificationService.notify(post) }
end

lib/my_engine.rb:

require "my_engine/engine"

module MyEngine
  mattr_accessor :default_locale, :max_items_per_page, :enable_caching
  mattr_accessor :user_class, :admin_class, :after_create_post

  def self.configure
    yield self
  end

  # Resolve configured class names
  def self.user_class_constant
    user_class.constantize
  end

  def self.admin_class_constant
    admin_class.constantize
  end
end

Mounting Engines

Routes Configuration

Parent app config/routes.rb:

Rails.application.routes.draw do
  # Mount at root path
  mount MyEngine::Engine => "/"

  # Mount at specific path
  mount MyEngine::Engine => "/blog", as: "blog"

  # Mount with constraints
  mount MyEngine::Engine => "/admin", constraints: { subdomain: "admin" }

  # Multiple mounts
  mount BlogEngine::Engine => "/blog"
  mount ForumEngine::Engine => "/forum"
  mount ShopEngine::Engine => "/shop"
end

Engine Routes

Engine config/routes.rb:

MyEngine::Engine.routes.draw do
  root to: "posts#index"

  resources :posts do
    resources :comments
    member do
      post :publish
      post :archive
    end
  end

  resources :categories, only: [:index, :show]

  namespace :admin do
    resources :posts
    resources :settings
  end

  # Direct routes to parent app (use main_app)
  get "/profile", to: redirect { |params, request|
    main_app.profile_path
  }
end

Route Helpers

Accessing engine routes from parent app:

<!-- Parent app views -->
<%= link_to "Blog", my_engine.root_path %>
<%= link_to "New Post", my_engine.new_post_path %>
<%= link_to "Post", my_engine.post_path(@post) %>

<!-- With mount path -->
<%= link_to "Blog", blog.root_path %>
<%= link_to "All Posts", blog.posts_path %>

Accessing parent app routes from engine:

<!-- Engine views -->
<%= link_to "Home", main_app.root_path %>
<%= link_to "User Profile", main_app.user_path(@user) %>
<%= link_to "Settings", main_app.settings_path %>

In controllers:

module MyEngine
  class PostsController < ApplicationController
    def create
      @post = Post.create(post_params)
      if @post.persisted?
        redirect_to @post  # Engine route
      else
        render :new
      end
    end

    def back_to_app
      redirect_to main_app.root_path  # Parent app route
    end
  end
end

URL generation:

# In parent app
MyEngine::Engine.routes.url_helpers.post_path(post)
blog.post_path(post)  # If mounted as 'blog'

# In engine
main_app.user_path(user)
MyEngine::Engine.routes.url_helpers.post_path(post)

Isolation and Shared Resources

Accessing Parent App from Engine

Referencing parent app models:

module MyEngine
  class Post < ApplicationRecord
    # Option 1: Direct reference (breaks isolation)
    belongs_to :user, class_name: "::User"

    # Option 2: Configured class (better)
    def user_class
      MyEngine.user_class_constant
    end

    def user
      user_class.find(user_id)
    end
  end
end

Calling parent app services:

module MyEngine
  class PostsController < ApplicationController
    def create
      @post = Post.create(post_params)

      # Call parent app service
      ::NotificationService.notify_new_post(@post)

      # Or use configured callback
      MyEngine.after_create_post&.call(@post)

      redirect_to @post
    end
  end
end

Using parent app helpers:

module MyEngine
  class ApplicationController < ActionController::Base
    helper_method :current_user

    def current_user
      # Delegate to parent app
      main_app.current_user if main_app.respond_to?(:current_user)
    end
  end
end

Accessing Engine from Parent App

Using engine models:

# Parent app controller
class DashboardController < ApplicationController
  def index
    @recent_posts = MyEngine::Post.recent.limit(5)
    @post_count = MyEngine::Post.count
  end
end

Overriding engine controllers:

# Parent app: app/controllers/my_engine/posts_controller.rb
module MyEngine
  class PostsController < MyEngine::PostsController
    # Override specific action
    def index
      @posts = Post.where(published: true).by_user(current_user)
      render "my_engine/posts/index"
    end

    # Add new action
    def featured
      @posts = Post.featured
    end
  end
end

Decorating engine models:

# Parent app: app/decorators/my_engine/post_decorator.rb
MyEngine::Post.class_eval do
  # Add associations
  has_many :likes, class_name: "::Like", as: :likeable

  # Add methods
  def featured?
    featured_at.present? && featured_at > 30.days.ago
  end

  # Override methods
  def display_title
    published? ? title : "[Draft] #{title}"
  end
end

Loading decorators in engine:

# Engine: lib/my_engine/engine.rb
module MyEngine
  class Engine < ::Rails::Engine
    config.to_prepare do
      # Load parent app decorators
      decorator_path = Rails.root.join("app", "decorators", "my_engine")
      if decorator_path.exist?
        Dir.glob(decorator_path.join("**", "*_decorator.rb")).each do |file|
          Rails.configuration.cache_classes ? require(file) : load(file)
        end
      end
    end
  end
end

Shared Concerns

Engine concern:

# Engine: app/models/concerns/my_engine/publishable.rb
module MyEngine
  module Publishable
    extend ActiveSupport::Concern

    included do
      scope :published, -> { where.not(published_at: nil) }
      scope :draft, -> { where(published_at: nil) }
    end

    def publish!
      update(published_at: Time.current)
    end

    def published?
      published_at.present?
    end
  end
end

Using in parent app:

# Parent app model
class Article < ApplicationRecord
  include MyEngine::Publishable

  # Article now has publish!, published?, published, draft scopes
end

Migrations

Creating Migrations

Generate migration in engine:

cd engines/my_engine
rails generate migration CreatePosts title:string body:text published:boolean

Generated migration:

# engines/my_engine/db/migrate/20260206120000_create_my_engine_posts.rb
class CreateMyEnginePosts < ActiveRecord::Migration[7.0]
  def change
    create_table :my_engine_posts do |t|
      t.string :title
      t.text :body
      t.boolean :published, default: false

      t.timestamps
    end

    add_index :my_engine_posts, :published
  end
end

Installing Migrations

Copy migrations to parent app:

# From parent app directory
rake my_engine:install:migrations

# Or for all engines
rake railties:install:migrations

What happens:

Copies migrations from engine to parent app db/migrate/
Prefixes with timestamp to maintain order
Adds scope comment to track origin

Result:

# Parent app: db/migrate/20260206120000_create_my_engine_posts.my_engine.rb
# This migration comes from my_engine (originally 20260206120000)
class CreateMyEnginePosts < ActiveRecord::Migration[7.0]
  def change
    create_table :my_engine_posts do |t|
      t.string :title
      t.text :body
      t.boolean :published, default: false

      t.timestamps
    end

    add_index :my_engine_posts, :published
  end
end

Run migrations:

rake db:migrate

Migration Conflicts

Problem: Same migration copied twice

Solution 1: Check before copying

# Skip if already copied
rake my_engine:install:migrations SKIP_EXISTING=true

Solution 2: Use version control

# Engine: lib/tasks/install.rake
namespace :my_engine do
  namespace :install do
    desc "Install MyEngine migrations"
    task :migrations_with_check do
      # Check if migrations already installed
      existing = Dir.glob(Rails.root.join("db/migrate/*my_engine.rb"))
      if existing.any?
        puts "Migrations already installed. Skipping."
      else
        Rake::Task["my_engine:install:migrations"].invoke
      end
    end
  end
end

Referencing Parent App Tables

Foreign keys to parent app:

class AddUserRefToMyEnginePosts < ActiveRecord::Migration[7.0]
  def change
    add_reference :my_engine_posts, :user, foreign_key: true
  end
end

Polymorphic associations:

class CreateMyEngineComments < ActiveRecord::Migration[7.0]
  def change
    create_table :my_engine_comments do |t|
      t.references :commentable, polymorphic: true, null: false
      t.text :body

      t.timestamps
    end
  end
end

Rollback Strategies

Rolling back engine migrations:

# Rollback last migration
rake db:rollback

# Rollback specific version
rake db:migrate:down VERSION=20260206120000

# Rollback all engine migrations
rake db:migrate:down VERSION=<first_engine_migration_version>

Tracking migration status:

rake db:migrate:status

Generators

Custom Generators

Engine generator structure:

engines/my_engine/lib/generators/
└── my_engine/
    ├── install/
    │   ├── install_generator.rb
    │   └── templates/
    │       └── initializer.rb
    └── post/
        ├── post_generator.rb
        └── templates/
            └── post.rb.erb

Install Generator

engines/my_engine/lib/generators/my_engine/install/install_generator.rb:

module MyEngine
  module Generators
    class InstallGenerator < Rails::Generators::Base
      source_root File.expand_path("templates", __dir__)

      desc "Creates MyEngine initializer and copies migrations"

      def copy_initializer
        template "initializer.rb", "config/initializers/my_engine.rb"
      end

      def copy_migrations
        rake "my_engine:install:migrations"
      end

      def mount_engine
        route 'mount MyEngine::Engine => "/blog"'
      end

      def show_readme
        readme "README" if behavior == :invoke
      end
    end
  end
end

templates/initializer.rb:

MyEngine.configure do |config|
  # Configure user class
  config.user_class = "User"

  # Configure pagination
  config.max_items_per_page = 25

  # Configure callbacks
  # config.after_create_post = ->(post) {
  #   # Your custom logic here
  # }
end

Usage:

rails generate my_engine:install

Model Generator

engines/my_engine/lib/generators/my_engine/post/post_generator.rb:

module MyEngine
  module Generators
    class PostGenerator < Rails::Generators::NamedBase
      source_root File.expand_path("templates", __dir__)

      argument :attributes, type: :array, default: [], banner: "field:type field:type"

      def create_model_file
        template "post.rb.erb", "app/models/my_engine/#{file_name}.rb"
      end

      def create_migration_file
        migration_template "migration.rb.erb",
                          "db/migrate/create_my_engine_#{table_name}.rb",
                          migration_version: migration_version
      end

      private

      def migration_version
        "[#{Rails::VERSION::MAJOR}.#{Rails::VERSION::MINOR}]"
      end

      def attributes_hash
        return "{}" if attributes.empty?
        attributes.map { |attr| "#{attr.name}: #{attr.type}" }.join(", ")
      end
    end
  end
end

templates/post.rb.erb:

module MyEngine
  class <%= class_name %> < ApplicationRecord
    # Validations
    validates :title, presence: true

    # Associations
    belongs_to :user, class_name: MyEngine.user_class

    # Scopes
    scope :recent, -> { order(created_at: :desc) }

    # Instance methods
    def to_s
      title
    end
  end
end

Usage:

rails generate my_engine:post Article title:string body:text

Scaffold Generator

engines/my_engine/lib/generators/my_engine/scaffold/scaffold_generator.rb:

module MyEngine
  module Generators
    class ScaffoldGenerator < Rails::Generators::NamedBase
      source_root File.expand_path("templates", __dir__)

      argument :attributes, type: :array, default: []

      def create_model
        invoke "my_engine:post", [name] + attributes.map(&:to_s)
      end

      def create_controller
        template "controller.rb.erb",
                 "app/controllers/my_engine/#{file_name.pluralize}_controller.rb"
      end

      def create_views
        %w[index show new edit _form].each do |view|
          template "views/#{view}.html.erb",
                   "app/views/my_engine/#{file_name.pluralize}/#{view}.html.erb"
        end
      end

      def add_routes
        route "resources :#{file_name.pluralize}"
      end
    end
  end
end

Generator Hooks

Customize generator behavior:

module MyEngine
  class Engine < ::Rails::Engine
    config.generators do |g|
      g.test_framework :rspec, fixture: false
      g.fixture_replacement :factory_bot, dir: "spec/factories"
      g.assets false
      g.helper false
    end
  end
end

Testing Engines

Test Structure

Engine test directory:

engines/my_engine/
├── spec/
│   ├── controllers/
│   │   └── my_engine/
│   ├── models/
│   │   └── my_engine/
│   ├── features/
│   ├── factories/
│   ├── support/
│   ├── rails_helper.rb
│   └── spec_helper.rb
└── test/
    └── dummy/  # Dummy Rails app for testing
        ├── app/
        ├── config/
        └── db/

Dummy Application

test/dummy/config/application.rb:

module Dummy
  class Application < Rails::Application
    config.load_defaults 7.0

    # Configure for testing
    config.eager_load = false
    config.consider_all_requests_local = true
    config.action_controller.perform_caching = false
  end
end

test/dummy/config/routes.rb:

Rails.application.routes.draw do
  mount MyEngine::Engine => "/my_engine"
end

RSpec Configuration

spec/rails_helper.rb:

ENV['RAILS_ENV'] ||= 'test'

require File.expand_path('../test/dummy/config/environment', __dir__)

abort("Running in production mode!") if Rails.env.production?

require 'rspec/rails'
require 'factory_bot_rails'
require 'capybara/rspec'

# Load support files
Dir[MyEngine::Engine.root.join('spec/support/**/*.rb')].each { |f| require f }

RSpec.configure do |config|
  config.use_transactional_fixtures = true
  config.infer_spec_type_from_file_location!
  config.filter_rails_from_backtrace!

  # FactoryBot
  config.include FactoryBot::Syntax::Methods

  # Engine routes
  config.include MyEngine::Engine.routes.url_helpers

  # Dummy app routes
  config.include Rails.application.routes.url_helpers, type: :controller
end

Model Specs

spec/models/my_engine/post_spec.rb:

require 'rails_helper'

module MyEngine
  RSpec.describe Post, type: :model do
    describe "validations" do
      it { should validate_presence_of(:title) }
      it { should validate_presence_of(:body) }
    end

    describe "associations" do
      it { should belong_to(:user) }
      it { should have_many(:comments) }
    end

    describe "scopes" do
      let!(:published_post) { create(:my_engine_post, published_at: 1.day.ago) }
      let!(:draft_post) { create(:my_engine_post, published_at: nil) }

      describe ".published" do
        it "returns only published posts" do
          expect(Post.published).to include(published_post)
          expect(Post.published).not_to include(draft_post)
        end
      end

      describe ".draft" do
        it "returns only draft posts" do
          expect(Post.draft).to include(draft_post)
          expect(Post.draft).not_to include(published_post)
        end
      end
    end

    describe "#publish!" do
      let(:post) { create(:my_engine_post, published_at: nil) }

      it "sets published_at" do
        expect { post.publish! }.to change { post.published_at }.from(nil)
      end

      it "marks post as published" do
        post.publish!
        expect(post).to be_published
      end
    end
  end
end

Controller Specs

spec/controllers/my_engine/posts_controller_spec.rb:

require 'rails_helper'

module MyEngine
  RSpec.describe PostsController, type: :controller do
    routes { MyEngine::Engine.routes }

    let(:user) { create(:user) }
    let(:post_attrs) { attributes_for(:my_engine_post) }

    before { sign_in user }

    describe "GET #index" do
      let!(:posts) { create_list(:my_engine_post, 3) }

      it "returns success response" do
        get :index
        expect(response).to be_successful
      end

      it "assigns @posts" do
        get :index
        expect(assigns(:posts)).to match_array(posts)
      end
    end

    describe "GET #show" do
      let(:post) { create(:my_engine_post) }

      it "returns success response" do
        get :show, params: { id: post.id }
        expect(response).to be_successful
      end
    end

    describe "POST #create" do
      context "with valid params" do
        it "creates a new Post" do
          expect {
            post :create, params: { post: post_attrs }
          }.to change(Post, :count).by(1)
        end

        it "redirects to the created post" do
          post :create, params: { post: post_attrs }
          expect(response).to redirect_to(Post.last)
        end
      end

      context "with invalid params" do
        let(:invalid_attrs) { { title: "" } }

        it "does not create post" do
          expect {
            post :create, params: { post: invalid_attrs }
          }.not_to change(Post, :count)
        end

        it "renders new template" do
          post :create, params: { post: invalid_attrs }
          expect(response).to render_template(:new)
        end
      end
    end
  end
end

Feature Specs

spec/features/my_engine/posts_spec.rb:

require 'rails_helper'

module MyEngine
  RSpec.feature "Posts", type: :feature do
    let(:user) { create(:user) }

    before { login_as(user) }

    scenario "User creates a new post" do
      visit my_engine.root_path
      click_link "New Post"

      fill_in "Title", with: "My First Post"
      fill_in "Body", with: "This is the post body"
      click_button "Create Post"

      expect(page).to have_content("Post was successfully created")
      expect(page).to have_content("My First Post")
    end

    scenario "User edits a post" do
      post = create(:my_engine_post, user: user)

      visit my_engine.post_path(post)
      click_link "Edit"

      fill_in "Title", with: "Updated Title"
      click_button "Update Post"

      expect(page).to have_content("Post was successfully updated")
      expect(page).to have_content("Updated Title")
    end

    scenario "User publishes a draft post" do
      post = create(:my_engine_post, user: user, published_at: nil)

      visit my_engine.post_path(post)
      click_button "Publish"

      expect(page).to have_content("Post published")
      expect(post.reload).to be_published
    end
  end
end

Testing with Parent App

Integration testing:

# Parent app: spec/features/blog_integration_spec.rb
require 'rails_helper'

RSpec.feature "Blog Integration", type: :feature do
  let(:user) { create(:user) }

  scenario "User navigates from home to blog" do
    visit root_path
    click_link "Blog"

    expect(current_path).to eq(blog.root_path)
    expect(page).to have_content("Blog Posts")
  end

  scenario "Creating post sends notification" do
    login_as(user)

    expect(NotificationService).to receive(:notify_new_post)

    visit blog.new_post_path
    fill_in "Title", with: "Test Post"
    fill_in "Body", with: "Test body"
    click_button "Create Post"
  end
end

Factory Bot Setup

spec/factories/my_engine/posts.rb:

FactoryBot.define do
  factory :my_engine_post, class: 'MyEngine::Post' do
    sequence(:title) { |n| "Post #{n}" }
    body { "This is the post body" }
    published_at { 1.day.ago }
    association :user

    trait :draft do
      published_at { nil }
    end

    trait :with_comments do
      after(:create) do |post|
        create_list(:my_engine_comment, 3, post: post)
      end
    end
  end
end

Dependencies Management

Gemspec Configuration

my_engine.gemspec:

$:.push File.expand_path("lib", __dir__)

require "my_engine/version"

Gem::Specification.new do |spec|
  spec.name        = "my_engine"
  spec.version     = MyEngine::VERSION
  spec.authors     = ["Your Name"]
  spec.email       = ["your.email@example.com"]
  spec.homepage    = "https://github.com/yourname/my_engine"
  spec.summary     = "A Rails engine for blog functionality"
  spec.description = "Provides blog posts, comments, and categories"
  spec.license     = "MIT"

  spec.files = Dir[
    "{app,config,db,lib}/**/*",
    "MIT-LICENSE",
    "Rakefile",
    "README.md"
  ]

  # Rails dependency
  spec.add_dependency "rails", ">= 7.0.0"

  # Additional dependencies
  spec.add_dependency "kaminari", "~> 1.2"  # Pagination
  spec.add_dependency "redcarpet", "~> 3.5"  # Markdown
  spec.add_dependency "pundit", "~> 2.3"  # Authorization

  # Development dependencies
  spec.add_development_dependency "rspec-rails", "~> 6.0"
  spec.add_development_dependency "factory_bot_rails", "~> 6.2"
  spec.add_development_dependency "capybara", "~> 3.39"
  spec.add_development_dependency "sqlite3", "~> 1.6"
  spec.add_development_dependency "puma", "~> 6.0"
end

Parent App Requirements

Parent app Gemfile:

# Local development
gem 'my_engine', path: 'engines/my_engine'

# Git repository
gem 'my_engine', git: 'https://github.com/yourname/my_engine.git'

# Rubygems (published)
gem 'my_engine', '~> 1.0'

# Multiple engines
gem 'blog_engine', path: 'engines/blog_engine'
gem 'forum_engine', path: 'engines/forum_engine'
gem 'shop_engine', path: 'engines/shop_engine'

Inter-Engine Dependencies

Engine depending on another engine:

# shop_engine.gemspec
spec.add_dependency "blog_engine", "~> 1.0"

Using shared engine:

# In shop_engine
module ShopEngine
  class Product < ApplicationRecord
    # Use blog engine for product articles
    has_many :blog_posts,
             class_name: "BlogEngine::Post",
             foreign_key: :product_id
  end
end

Shared configuration:

# lib/my_engine/engine.rb
module MyEngine
  class Engine < ::Rails::Engine
    # Require other engines
    require 'shared_engine'

    config.after_initialize do
      # Ensure other engines are loaded
      unless defined?(SharedEngine::Engine)
        raise "MyEngine requires SharedEngine"
      end
    end
  end
end

Configuration and Initialization

Initializers

Engine initializers:

# lib/my_engine/engine.rb
module MyEngine
  class Engine < ::Rails::Engine
    isolate_namespace MyEngine

    # Run before other initializers
    initializer "my_engine.early_setup", before: :load_config_initializers do
      # Setup early configuration
    end

    # Assets precompilation
    initializer "my_engine.assets.precompile" do |app|
      app.config.assets.precompile += %w(
        my_engine/application.js
        my_engine/application.css
        my_engine/admin.js
        my_engine/admin.css
      )
    end

    # Load decorators
    initializer "my_engine.load_decorators" do
      config.to_prepare do
        Dir.glob(Engine.root.join("app", "decorators", "**", "*_decorator.rb")).each do |c|
          Rails.configuration.cache_classes ? require(c) : load(c)
        end
      end
    end

    # Add middleware
    initializer "my_engine.middleware" do |app|
      app.middleware.use MyEngine::Middleware::TrackingMiddleware
    end

    # Register mime types
    initializer "my_engine.mime_types" do
      Mime::Type.register "application/vnd.my_engine+json", :my_engine_json
    end

    # Subscribe to ActiveSupport notifications
    initializer "my_engine.notifications" do
      ActiveSupport::Notifications.subscribe("post.created") do |*args|
        event = ActiveSupport::Notifications::Event.new(*args)
        # Handle event
      end
    end
  end
end

Environment-Specific Config

Engine configuration per environment:

# lib/my_engine/engine.rb
module MyEngine
  class Engine < ::Rails::Engine
    config.before_configuration do
      # Load environment-specific config
      config_file = Engine.root.join("config", "environments", "#{Rails.env}.rb")
      load(config_file) if File.exist?(config_file)
    end
  end
end

config/environments/production.rb:

MyEngine.configure do |config|
  config.enable_caching = true
  config.cache_store = :redis_cache_store
  config.log_level = :info
end

config/environments/development.rb:

MyEngine.configure do |config|
  config.enable_caching = false
  config.log_level = :debug
end

Secrets and Credentials

Accessing parent app credentials:

module MyEngine
  class ApiClient
    def api_key
      Rails.application.credentials.my_engine[:api_key]
    end
  end
end

Parent app credentials.yml.enc:

my_engine:
  api_key: abc123xyz
  api_secret: secret456

When to Use Engines

Use Cases for Engines

1. Reusable Components

Authentication systems (like Devise)
Admin panels (like ActiveAdmin)
CMS functionality
E-commerce platforms
Forum systems
Multi-tenancy

2. Application Modularization

Large monolith breakup
Feature-based organization
Team-based code ownership
Gradual microservices transition

3. Client Customization

White-label products
Configurable base functionality
Client-specific overrides

4. Multi-Application Sharing

Common functionality across apps
Company-wide components
API clients as engines

Engines vs Gems

When to use Engine:

Needs Rails integration (models, controllers, views)
Requires database tables
Has routes and UI
Needs to be mounted in parent app
Tightly coupled with Rails

When to use Gem:

Pure Ruby functionality
No Rails dependencies needed
Utility functions
Service clients
No database interaction

Example decisions:

Requirement	Solution
Add blog to app	Engine (models, controllers, views)
HTTP client	Gem (no Rails needed)
Admin dashboard	Engine (routes, UI, models)
Date formatting	Gem (utility function)
Forum system	Engine (complex Rails features)
API wrapper	Gem (unless needs AR models)

Anti-Patterns

Don't use engines when:

Single app, small feature - Just use regular Rails structure
No reusability planned - Overhead not worth it
Frequently changing boundaries - Modular Rails folders sufficient
Tight coupling required - Engine isolation becomes burden
Simple gem would work - Don't add Rails overhead unnecessarily

Warning signs of over-engineering:

# Bad: Too many small engines
engines/
├── user_profile_engine/  # Just 1 model
├── settings_engine/      # Just 1 controller
├── notification_engine/  # Could be a service object
└── email_engine/         # Could be ActionMailer

# Better: Reasonable engine boundaries
engines/
├── accounts_engine/      # User management, profiles, settings
└── messaging_engine/     # Notifications, emails, chat

Good engine boundaries:

Clear domain boundary
Minimal coupling with parent app
Self-contained functionality
Reusable or extractable
Team ownership alignment

MVC Architecture

Models (Active Record)

Model Basics

Basic model:

class User < ApplicationRecord
  # Validations
  validates :email, presence: true, uniqueness: true
  validates :name, presence: true
  validates :age, numericality: { greater_than: 0 }

  # Associations
  has_many :posts, dependent: :destroy
  has_many :comments
  has_one :profile, dependent: :destroy

  # Callbacks
  before_save :normalize_email
  after_create :send_welcome_email

  # Scopes
  scope :active, -> { where(active: true) }
  scope :recent, -> { order(created_at: :desc) }

  private

  def normalize_email
    self.email = email.downcase.strip
  end

  def send_welcome_email
    UserMailer.welcome(self).deliver_later
  end
end

Associations

All association types:

class User < ApplicationRecord
  # One-to-many
  has_many :posts
  has_many :comments

  # One-to-many with options
  has_many :published_posts,
           -> { where(published: true) },
           class_name: "Post"

  # Many-to-many with join table
  has_many :group_memberships
  has_many :groups, through: :group_memberships

  # One-to-one
  has_one :profile, dependent: :destroy
  has_one :address, through: :profile

  # Polymorphic
  has_many :comments, as: :commentable
end

class Post < ApplicationRecord
  belongs_to :user
  has_many :comments, as: :commentable, dependent: :destroy
  has_many :taggings
  has_many :tags, through: :taggings
end

class Comment < ApplicationRecord
  belongs_to :user
  belongs_to :commentable, polymorphic: true
end

Validations

Common validations:

class User < ApplicationRecord
  validates :email,
            presence: true,
            uniqueness: { case_sensitive: false },
            format: { with: URI::MailTo::EMAIL_REGEXP }

  validates :password,
            length: { minimum: 8 },
            if: :password_required?

  validates :age,
            numericality: {
              greater_than_or_equal_to: 18,
              less_than: 120
            }

  validates :username,
            presence: true,
            uniqueness: true,
            length: { in: 3..20 },
            format: {
              with: /\A[a-zA-Z0-9_]+\z/,
              message: "only letters, numbers, and underscores"
            }

  validates :terms_of_service,
            acceptance: true

  validates :role,
            inclusion: { in: %w[admin user guest] }

  validate :custom_validation

  private

  def password_required?
    new_record? || password.present?
  end

  def custom_validation
    if username.present? && username.start_with?("admin")
      errors.add(:username, "cannot start with 'admin'")
    end
  end
end

Callbacks

Callback order:

class Post < ApplicationRecord
  # Creation callbacks
  before_validation :set_defaults
  after_validation :log_validation
  before_save :prepare_content
  around_save :log_save
  before_create :set_publication_date
  after_create :notify_subscribers
  after_save :clear_cache
  after_commit :index_for_search, on: :create

  # Update callbacks
  before_update :check_changes
  after_update :notify_if_published

  # Destruction callbacks
  before_destroy :check_dependencies
  after_destroy :cleanup_assets

  private

  def set_defaults
    self.status ||= "draft"
  end

  def prepare_content
    self.content = ContentProcessor.process(content)
  end

  def log_save
    Rails.logger.info "Saving post #{id}"
    yield
    Rails.logger.info "Saved post #{id}"
  end

  def notify_subscribers
    NotificationJob.perform_later(id)
  end
end

Scopes and Queries

Scope patterns:

class Post < ApplicationRecord
  # Basic scopes
  scope :published, -> { where(published: true) }
  scope :draft, -> { where(published: false) }
  scope :recent, -> { order(created_at: :desc) }

  # Scopes with parameters
  scope :by_author, ->(author) { where(author: author) }
  scope :created_after, ->(date) { where("created_at > ?", date) }
  scope :with_tag, ->(tag) { joins(:tags).where(tags: { name: tag }) }

  # Chainable scopes
  scope :popular, -> { where("views > ?", 1000) }
  scope :recent_popular, -> { recent.popular }

  # Default scope (use cautiously)
  default_scope { where(deleted_at: nil) }

  # Class methods as scopes
  def self.search(query)
    where("title LIKE ? OR content LIKE ?", "%#{query}%", "%#{query}%")
  end
end

# Usage
Post.published.recent.limit(10)
Post.by_author("John").created_after(1.week.ago)

Controllers

Controller Basics

RESTful controller:

class PostsController < ApplicationController
  before_action :authenticate_user!
  before_action :set_post, only: [:show, :edit, :update, :destroy]
  before_action :authorize_post, only: [:edit, :update, :destroy]

  def index
    @posts = Post.published.recent.page(params[:page])
  end

  def show
    @comments = @post.comments.includes(:user)
  end

  def new
    @post = current_user.posts.build
  end

  def create
    @post = current_user.posts.build(post_params)

    if @post.save
      redirect_to @post, notice: "Post created successfully"
    else
      render :new, status: :unprocessable_entity
    end
  end

  def edit
  end

  def update
    if @post.update(post_params)
      redirect_to @post, notice: "Post updated successfully"
    else
      render :edit, status: :unprocessable_entity
    end
  end

  def destroy
    @post.destroy
    redirect_to posts_url, notice: "Post deleted successfully"
  end

  private

  def set_post
    @post = Post.find(params[:id])
  end

  def authorize_post
    unless @post.user == current_user || current_user.admin?
      redirect_to root_path, alert: "Not authorized"
    end
  end

  def post_params
    params.require(:post).permit(:title, :content, :published, tag_ids: [])
  end
end

Concerns

Controller concern:

# app/controllers/concerns/paginatable.rb
module Paginatable
  extend ActiveSupport::Concern

  included do
    helper_method :page, :per_page
  end

  def page
    params[:page] || 1
  end

  def per_page
    params[:per_page] || 25
  end

  def paginate(collection)
    collection.page(page).per(per_page)
  end
end

# Usage in controller
class PostsController < ApplicationController
  include Paginatable

  def index
    @posts = paginate(Post.published)
  end
end

Strong Parameters

Nested parameters:

class PostsController < ApplicationController
  private

  def post_params
    params.require(:post).permit(
      :title,
      :content,
      :published,
      tag_ids: [],
      comments_attributes: [:id, :body, :_destroy],
      metadata: [:description, :keywords]
    )
  end
end

Views

View Helpers

Custom helpers:

# app/helpers/application_helper.rb
module ApplicationHelper
  def formatted_date(date)
    date.strftime("%B %d, %Y") if date
  end

  def user_avatar(user, size: 40)
    image_tag user.avatar_url, size: "#{size}x#{size}", class: "avatar"
  end

  def markdown(text)
    return "" unless text

    markdown = Redcarpet::Markdown.new(
      Redcarpet::Render::HTML,
      autolink: true,
      tables: true
    )
    markdown.render(text).html_safe
  end
end

Partials

Using partials:

<!-- app/views/posts/index.html.erb -->
<h1>Posts</h1>

<div class="posts">
  <%= render @posts %>
</div>

<!-- app/views/posts/_post.html.erb -->
<div class="post">
  <h2><%= link_to post.title, post %></h2>
  <p><%= post.excerpt %></p>
  <%= render "shared/post_meta", post: post %>
</div>

<!-- app/views/shared/_post_meta.html.erb -->
<div class="post-meta">
  <span>By <%= post.author.name %></span>
  <span><%= formatted_date(post.created_at) %></span>
</div>

Query Optimization

N+1 Queries

Problem:

# Bad - N+1 queries
@posts = Post.all
@posts.each do |post|
  puts post.user.name  # Additional query for each post
end

Solution:

# Good - Eager loading
@posts = Post.includes(:user)
@posts.each do |post|
  puts post.user.name  # No additional queries
end

Eager Loading

Different loading strategies:

# includes - Uses LEFT OUTER JOIN or separate queries
Post.includes(:user, :comments)

# preload - Always uses separate queries
Post.preload(:user, :comments)

# eager_load - Always uses LEFT OUTER JOIN
Post.eager_load(:user, :comments)

# Nested associations
Post.includes(comments: :user)

# Multiple associations
Post.includes(:user, :tags, comments: [:user, :likes])

Select and Pluck

Optimize queries:

# Select specific columns
Post.select(:id, :title, :created_at)

# Pluck for single values
Post.pluck(:title)  # Returns array of titles
Post.pluck(:id, :title)  # Returns array of arrays

# Distinct
Post.select(:author_id).distinct

# Count without loading records
Post.where(published: true).count

Indexes

Migration with indexes:

class AddIndexesToPosts < ActiveRecord::Migration[7.0]
  def change
    add_index :posts, :user_id
    add_index :posts, :published
    add_index :posts, [:user_id, :created_at]
    add_index :posts, :title, unique: true
  end
end

Service Objects

When to use service objects:

Complex business logic spanning multiple models
Operations requiring external API calls
Multi-step processes with transactions
Logic that doesn't fit cleanly in models/controllers

Service object pattern:

# app/services/posts/publish_service.rb
module Posts
  class PublishService
    def initialize(post, user)
      @post = post
      @user = user
    end

    def call
      return failure("Already published") if @post.published?
      return failure("Not authorized") unless can_publish?

      ActiveRecord::Base.transaction do
        @post.update!(published: true, published_at: Time.current)
        notify_subscribers
        index_for_search
      end

      success(@post)
    rescue => e
      failure(e.message)
    end

    private

    def can_publish?
      @user.admin? || @post.user == @user
    end

    def notify_subscribers
      NotificationJob.perform_later(@post.id)
    end

    def index_for_search
      SearchIndexJob.perform_later(@post.id)
    end

    def success(data)
      OpenStruct.new(success?: true, data: data, error: nil)
    end

    def failure(error)
      OpenStruct.new(success?: false, data: nil, error: error)
    end
  end
end

# Usage in controller
def publish
  result = Posts::PublishService.new(@post, current_user).call

  if result.success?
    redirect_to result.data, notice: "Published successfully"
  else
    redirect_to @post, alert: result.error
  end
end

Form Objects

Complex form handling:

# app/forms/user_registration_form.rb
class UserRegistrationForm
  include ActiveModel::Model

  attr_accessor :email, :password, :password_confirmation
  attr_accessor :company_name, :company_address

  validates :email, presence: true, format: { with: URI::MailTo::EMAIL_REGEXP }
  validates :password, presence: true, length: { minimum: 8 }
  validates :password_confirmation, presence: true
  validates :company_name, presence: true

  validate :passwords_match

  def save
    return false unless valid?

    ActiveRecord::Base.transaction do
      @user = User.create!(
        email: email,
        password: password
      )

      @company = Company.create!(
        name: company_name,
        address: company_address,
        owner: @user
      )

      @user.update!(company: @company)
    end

    true
  rescue => e
    errors.add(:base, e.message)
    false
  end

  attr_reader :user, :company

  private

  def passwords_match
    if password != password_confirmation
      errors.add(:password_confirmation, "doesn't match password")
    end
  end
end

# Controller
def create
  @form = UserRegistrationForm.new(registration_params)

  if @form.save
    redirect_to @form.user, notice: "Registration successful"
  else
    render :new
  end
end

Decorators/Presenters

Decorator pattern:

# app/decorators/post_decorator.rb
class PostDecorator < SimpleDelegator
  include ActionView::Helpers::TextHelper
  include ActionView::Helpers::UrlHelper

  def formatted_date
    created_at.strftime("%B %d, %Y")
  end

  def excerpt(length: 200)
    truncate(content, length: length)
  end

  def author_link
    link_to author.name, author
  end

  def status_badge
    published? ? "Published" : "Draft"
  end

  def reading_time
    words = content.split.size
    minutes = (words / 200.0).ceil
    "#{minutes} min read"
  end
end

# Usage
@post = PostDecorator.new(Post.find(params[:id]))

Background Jobs

ActiveJob

Job structure:

# app/jobs/notification_job.rb
class NotificationJob < ApplicationJob
  queue_as :default

  retry_on StandardError, wait: 5.seconds, attempts: 3
  discard_on ActiveJob::DeserializationError

  def perform(post_id)
    post = Post.find(post_id)
    post.subscribers.each do |subscriber|
      UserMailer.new_post(subscriber, post).deliver_now
    end
  end
end

# Enqueue job
NotificationJob.perform_later(post.id)
NotificationJob.set(wait: 1.hour).perform_later(post.id)
NotificationJob.set(queue: :urgent).perform_later(post.id)

Sidekiq

Sidekiq worker:

class HardWorker
  include Sidekiq::Worker
  sidekiq_options retry: 5, queue: :default

  def perform(post_id, user_id)
    post = Post.find(post_id)
    user = User.find(user_id)

    # Process heavy work
  end
end

# Enqueue
HardWorker.perform_async(post.id, user.id)
HardWorker.perform_in(1.hour, post.id, user.id)
HardWorker.perform_at(Time.now + 1.day, post.id, user.id)

Routing

Resources

RESTful routes:

Rails.application.routes.draw do
  # Standard resources
  resources :posts

  # Nested resources
  resources :posts do
    resources :comments
  end

  # Limited actions
  resources :posts, only: [:index, :show]
  resources :posts, except: [:destroy]

  # Member and collection routes
  resources :posts do
    member do
      post :publish
      post :archive
    end

    collection do
      get :search
      get :featured
    end
  end

  # Shallow nesting
  resources :posts, shallow: true do
    resources :comments
  end

  # Concerns
  concern :commentable do
    resources :comments
  end

  resources :posts, concerns: :commentable
  resources :articles, concerns: :commentable
end

Caching

Fragment caching:

<% cache @post do %>
  <div class="post">
    <h2><%= @post.title %></h2>
    <%= markdown @post.content %>
  </div>
<% end %>

Russian doll caching:

<% cache @post do %>
  <div class="post">
    <h2><%= @post.title %></h2>

    <% cache @post.comments do %>
      <% @post.comments.each do |comment| %>
        <% cache comment do %>
          <%= render comment %>
        <% end %>
      <% end %>
    <% end %>
  </div>
<% end %>

Security

Mass assignment protection:

params.require(:user).permit(:email, :name)

SQL injection prevention:

# Bad
User.where("name = '#{params[:name]}'")

# Good
User.where("name = ?", params[:name])
User.where(name: params[:name])

XSS prevention:

<!-- Escaped by default -->
<%= @post.title %>

<!-- Raw HTML (dangerous) -->
<%== @post.html_content %>
<%= raw @post.html_content %>
<%= @post.html_content.html_safe %>

CSRF protection:

class ApplicationController < ActionController::Base
  protect_from_forgery with: :exception
end

API Patterns

API controller:

module Api
  module V1
    class PostsController < ApiController
      def index
        @posts = Post.published.page(params[:page])
        render json: @posts, each_serializer: PostSerializer
      end

      def show
        @post = Post.find(params[:id])
        render json: @post, serializer: PostDetailSerializer
      end

      def create
        @post = current_user.posts.build(post_params)

        if @post.save
          render json: @post, status: :created
        else
          render json: { errors: @post.errors }, status: :unprocessable_entity
        end
      end
    end
  end
end

Serializer:

class PostSerializer < ActiveModel::Serializer
  attributes :id, :title, :excerpt, :created_at

  belongs_to :user
  has_many :tags

  def excerpt
    object.content.truncate(200)
  end
end

Best Practices

Keep controllers thin - Move logic to models/services
Use concerns for shared behavior - DRY across models/controllers
Eager load associations - Avoid N+1 queries
Use background jobs - For slow/external operations
Cache aggressively - Fragment and Russian doll caching
Index database columns - For frequently queried fields
Validate at multiple levels - Database, model, and form
Use transactions - For multi-step operations
Write tests - Models, controllers, features
Follow REST conventions - Standard resource routing

Anti-Patterns to Avoid

Fat controllers - Business logic in controllers
God objects - Models doing too much
Callback hell - Too many interdependent callbacks
N+1 queries - Missing eager loading
Skipping validations - Using save(validate: false)
Hard-coding - Magic numbers/strings instead of constants
Ignoring security - Raw SQL, mass assignment
Missing indexes - Slow queries on foreign keys
Premature optimization - Over-engineering simple features
Inconsistent naming - Breaking Rails conventions

Summary

Rails patterns provide structure for building maintainable applications. Rails Engines enable modular architecture for large applications and reusable components. Follow conventions, optimize queries, extract complex logic, and leverage Rails' powerful features for clean, efficient code.

The Rails Engines section provides comprehensive coverage of:

Creating mountable vs full engines
Engine structure and namespace isolation
Mounting engines and route configuration
Sharing functionality between engines and parent apps
Migration management and installation
Custom generators for engine components
Comprehensive testing strategies with RSpec
Dependency management in gemspecs
Configuration and initialization patterns
Decision frameworks for when to use engines

Use this knowledge to build modular, maintainable Rails applications and create powerful, reusable components.

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