Rails Quick Reference for Experienced Programmers

For more see:

• Getting Started with Rails

MVC Architecture

• Models (ActiveRecord): Database tables = Ruby classes. Associations (has_many, belongs_to), validations, and queries all in the model. Migrations version-control your schema.
• Controllers: HTTP routing → controller actions. Convention: UsersController#show maps to /users/:id. Actions render views or return JSON.
• Views: ERB templates (Ruby embedded in HTML). Layouts wrap views. Partials for reuse.

Convention Over Configuration

• File paths = class names: app/models/user.rb → User class
• Database tables are pluralized model names: User → users table
• Follow conventions → zero config. Fight them → pain.

Key Components

• Routes (config/routes.rb): RESTful by default. resources :users gives you 7 CRUD routes.
• ActiveRecord: ORM with chainable queries. User.where(active: true).order(:name) → SQL
• Migrations: Schema changes as Ruby code. Run with rails db:migrate
• Asset Pipeline (or Propshaft/Importmaps now): JS/CSS compilation/bundling
• Gems: Bundler manages dependencies via Gemfile

Routes Conventions

# config/routes.rb
resources :posts  # Creates 7 RESTful routes:

HTTP Verb	Path	Controller#Action	Purpose
GET	/posts	posts#index	List all
GET	/posts/new	posts#new	New form
POST	/posts	posts#create	Create
GET	/posts/:id	posts#show	Show one
GET	/posts/:id/edit	posts#edit	Edit form
PATCH/PUT	/posts/:id	posts#update	Update
DELETE	/posts/:id	posts#destroy	Delete

# Nested resources
resources :posts do
  resources :comments  # /posts/:post_id/comments
end

# Custom routes
get 'about', to: 'pages#about'
root 'posts#index'  # Homepage

# Namespace
namespace :admin do
  resources :users  # /admin/users → Admin::UsersController
end

CLI Essentials

rails new myapp          # Scaffold new app
rails generate model User name:string email:string
rails generate controller Users
rails db:migrate         # Run pending migrations
rails console            # IRB with app loaded
rails server             # Start dev server (port 3000)
rails routes             # Show all routes

Migration Commands

rails db:migrate                    # Run all pending migrations
rails db:rollback                   # Undo last migration
rails db:rollback STEP=3            # Undo last 3 migrations
rails db:migrate:down VERSION=20240101120000  # Undo a specific migration
rails db:migrate:up VERSION=20240101120000    # Run a specific migration
rails db:migrate VERSION=20240101120000       # Migrate to a specific version (up or down)
rails db:migrate:status             # Show status of all migrations (up/down)
rails db:migrate:redo               # Rollback then re-run last migration
rails db:migrate:redo STEP=3        # Rollback then re-run last 3 migrations
rails db:reset                      # Drop, recreate, and re-migrate the database
rails db:seed                       # Run db/seeds.rb

Directory Layout

• app/ - Your code (models, views, controllers)
• config/ - Routes, database.yml, application config
• db/ - Migrations, schema.rb, seeds.rb
• lib/ - Custom modules
• public/ - Static files
• test/ or spec/ - Tests

ActiveRecord Patterns

User.create(name: "Tom")
User.find(1)
User.where(admin: true).first
user.posts  # Association from has_many :posts

Associations

class User < ApplicationRecord
  has_many :posts                    # user.posts → looks for user_id in posts table
  has_many :comments, through: :posts # user.comments via join
  has_one :profile                   # user.profile → looks for user_id in profiles table
  has_and_belongs_to_many :roles     # join table: roles_users (alphabetical)
end

class Post < ApplicationRecord
  belongs_to :user                   # post.user → requires user_id column on posts table
  belongs_to :author, class_name: "User", foreign_key: "author_id"  # custom name
  has_many :comments, dependent: :destroy  # delete comments when post is deleted
end

Column Naming Conventions

• belongs_to :user expects a user_id column (integer/bigint) on the current model's table
• belongs_to :author, class_name: "User" expects an author_id column
• General rule: belongs_to :something → needs something_id column
• has_many/has_one don't add columns to the current table — the foreign key lives on the other table

Common Association Options

Option	Example	Purpose
dependent	has_many :posts, dependent: :destroy	What happens to children when parent is deleted (:destroy, :nullify, :restrict_with_error)
foreign_key	belongs_to :author, foreign_key: "author_id"	Specify non-conventional FK column
class_name	belongs_to :author, class_name: "User"	When association name differs from class
through	has_many :tags, through: :taggings	Join through another association
inverse_of	has_many :posts, inverse_of: :author	Helps Rails recognize bidirectional associations
optional	belongs_to :team, optional: true	Allow nil (belongs_to is required by default in Rails 5+)
counter_cache	belongs_to :user, counter_cache: true	Cache count in parent (needs posts_count column on users)

Migration for Associations

# Generate a migration that adds a foreign key
rails generate migration AddUserToPosts user:references

# Produces:
class AddUserToPosts < ActiveRecord::Migration[7.0]
  def change
    add_reference :posts, :user, null: false, foreign_key: true
    # Creates user_id column + index + DB-level foreign key constraint
  end
end

Modern Rails (~7.x)

• Hotwire (Turbo + Stimulus) instead of heavy JS frameworks
• Importmaps or esbuild for JS
• Tailwind integration common