Chapter 6: Models, Migrations, and the Database

Rails uses Active Record, an ORM (Object-Relational Mapping) that lets you work with database tables using plain Ruby objects. Every table has a model, and every change to the table structure is recorded in a migration.

In this chapter you will create a model, run migrations, and perform basic database operations (create, read, update, delete) from Ruby.

💾 Generating a Model

To create a model called Post with a title and a body, run:

rails generate model Post title:string body:text
rails db:migrate

The first command creates the model file app/models/post.rb and a migration. The db:migrate command applies the migration and creates the posts table in your database.

📜 Understanding Migrations

A migration describes a change to the database. Here is the migration created by the command above:

# db/migrate/xxx_create_posts.rb
class CreatePosts < ActiveRecord::Migration[7.0]
  def change
    create_table :posts do |t|
      t.string :title
      t.text :body

      t.timestamps
    end
  end
end

The t.string :title line creates a text column named title, and t.text :body creates a longer text column for the body.

🚀 CRUD with Active Record

Once the model exists, you can work with the database using Ruby:

# Create
post = Post.new(title: "Hello Rails", body: "My first post")
post.save

# Read
post = Post.find(1)
all_posts = Post.all

# Update
post.update(title: "Updated title")

# Delete
post.destroy

🔍 Using the Rails Console

To try these commands without a browser, open the Rails console:

rails console

Post.create(title: "Console post", body: "Created from the terminal")
Post.all.each { |p| puts p.title }

🏠 Practice Exercises

# 1. Create the Post model
rails generate model Post title:string body:text
rails db:migrate

# 2. In rails console, create and read records
rails console
p1 = Post.create(title: "First", body: "Hello")
p2 = Post.create(title: "Second", body: "World")
Post.count
Post.all.map(&:title)

In the next chapter you will build a complete welcome page using everything you have learned so far.