Database folder and migrations in Ruby on Rails - Mini Project: Build & Apply

Database Folder and Migrations in Rails

📖 Scenario: You are building a simple Rails application to manage a list of books. Each book has a title and an author.

🎯 Goal: Create the initial database setup by defining the database folder structure and writing a migration to create a books table with title and author columns.

📋 What You'll Learn

Create the db folder structure with migrate subfolder

Create a migration file to create a books table

Add title and author columns as strings in the migration

Write the migration class with change method

Ensure the migration file name follows Rails timestamp naming convention

💡 Why This Matters

🌍 Real World

Rails applications use migrations to manage database schema changes safely and consistently across development, testing, and production environments.

💼 Career

Understanding migrations is essential for Rails developers to create and modify database tables, enabling them to build data-driven web applications.

1

Create the database folder structure

Create a folder named db and inside it create a folder named migrate to hold migration files.

Ruby on Rails

# Create folders db and db/migrate in your Rails project directory
# Your code here

Hint

Rails stores migration files inside db/migrate folder.

2

Create a migration file for books table

Create a migration file named 20240101000000_create_books.rb inside db/migrate folder. The filename must start with a timestamp like 20240101000000 followed by _create_books.rb.

Ruby on Rails

# Create file 20240101000000_create_books.rb inside db/migrate
# Your code here

Hint

Migration files are Ruby classes inheriting from ActiveRecord::Migration.

3

Add columns to the books table in migration

Inside the change method of CreateBooks migration, write code to create a books table with string columns title and author using create_table :books do |t| block.

Ruby on Rails

# Add create_table block inside change method
# Your code here

Hint

Use create_table :books do |t| and inside the block add t.string :title and t.string :author.

4

Complete the migration file

Ensure the migration file 20240101000000_create_books.rb contains the full class CreateBooks inheriting from ActiveRecord::Migration[7.0] with the change method that creates the books table with title and author columns.

Ruby on Rails

# Verify the migration class and change method are complete
# Your code here

Hint

The migration class should be complete with the change method creating the table and columns.

Practice

1. What is the main purpose of the db/migrate folder in a Rails project?

easy

A. To store migration files that change the database structure step-by-step

B. To hold all the Ruby model files for the application

C. To keep the application configuration files

D. To store static assets like images and stylesheets

Solution

1. Step 1: Understand the role of the db/migrate folder

   This folder contains migration files that describe changes to the database schema over time.

2. Step 2: Differentiate from other folders

   Model files are in app/models, configs in config, and assets in app/assets.

3. Final Answer:

   To store migration files that change the database structure step-by-step -> Option A

4. Quick Check:

   Migration files = db/migrate folder [OK]

Hint: Migrations live in db/migrate to track database changes [OK]

Common Mistakes:

• Confusing migrations with model files
• Thinking assets or configs are in db/migrate
• Assuming db/migrate holds seed data

2. Which command correctly generates a new migration file in Rails?

easy

A. rails new migration AddAgeToUsers age:integer

B. rails generate migration AddAgeToUsers age:integer

C. rails create migration AddAgeToUsers age:integer

D. rails make migration AddAgeToUsers age:integer

Solution

1. Step 1: Recall the Rails command for generating migrations

   The correct command is rails generate migration followed by the migration name and fields.

2. Step 2: Check the options for correct syntax

   Only rails generate migration AddAgeToUsers age:integer uses the exact Rails command syntax for migration generation.

3. Final Answer:

   rails generate migration AddAgeToUsers age:integer -> Option B

4. Quick Check:

   Use rails generate migration to create migrations [OK]

Hint: Use 'rails generate migration' to create migration files [OK]

Common Mistakes:

• Using 'rails create' instead of 'rails generate'
• Confusing 'rails new' with migration commands
• Using 'rails make' which is invalid

3. Given this migration file content:

class CreateBooks < ActiveRecord::Migration[7.0]
  def change
    create_table :books do |t|
      t.string :title
      t.integer :pages
      t.timestamps
    end
  end
end

What will happen when you run rails db:migrate?

medium

A. Nothing happens because migrations only update existing tables

B. An error occurs because the migration syntax is wrong

C. The existing 'books' table will be deleted

D. A new table named 'books' with columns 'title', 'pages', and timestamps will be created

Solution

1. Step 1: Analyze the migration code

   The migration defines creating a new table called 'books' with columns 'title' (string), 'pages' (integer), and timestamps.

2. Step 2: Understand what rails db:migrate does

   It runs pending migrations, applying the changes to the database schema, here creating the 'books' table.

3. Final Answer:

   A new table named 'books' with columns 'title', 'pages', and timestamps will be created -> Option D

4. Quick Check:

   Create table migration runs successfully [OK]

Hint: Create_table migrations add new tables when migrated [OK]

Common Mistakes:

• Thinking migration deletes tables
• Assuming syntax error without checking code
• Believing migrations only update, not create

4. You wrote this migration:

class AddEmailToUsers < ActiveRecord::Migration
  def change
    add_column :users, :email, :string
  end
end

But running rails db:migrate gives an error: undefined method 'add_column'. What is the likely cause?

medium

A. The migration class should inherit from ActiveRecord::Migration[7.0] but it does

B. The migration file is missing the def change method

C. The migration class is missing the version number in inheritance

D. The migration file name does not match the class name

Solution

1. Step 1: Check the migration inheritance

   Rails 7 requires specifying the migration version in the class inheritance, e.g., ActiveRecord::Migration[7.0].

2. Step 2: Identify the error cause

   If the version is missing or incorrect, Rails may not recognize migration methods like add_column, causing the error.

3. Final Answer:

   The migration class is missing the version number in inheritance -> Option C

4. Quick Check:

   Migration version number required for method recognition [OK]

Hint: Always specify migration version in class inheritance [OK]

Common Mistakes:

• Ignoring migration version in class inheritance
• Assuming method missing means syntax error
• Not matching file name and class name

5. You want to rename a column username to login in the users table using a migration. Which migration code correctly does this?

hard

A. def change
rename_column :users, :username, :login
end

B. def change
change_column :users, :username, :login
end

C. def change
remove_column :users, :username
add_column :users, :login, :string
end

D. def change
rename_table :users, :login
end

Solution

1. Step 1: Understand the goal

   You want to rename a column, not change its type or remove and add a new one.

2. Step 2: Identify the correct migration method

   rename_column changes a column's name safely without losing data.

3. Step 3: Check options

   def change rename_column :users, :username, :login end uses rename_column :users, :username, :login, which is correct. def change change_column :users, :username, :login end wrongly uses change_column which changes type, not name. def change remove_column :users, :username add_column :users, :login, :string end removes and adds columns, which is risky. def change rename_table :users, :login end renames the whole table, not a column.

4. Final Answer:

   def change rename_column :users, :username, :login end -> Option A

5. Quick Check:

   Use rename_column to rename columns safely [OK]

Hint: Use rename_column to rename columns without data loss [OK]

Common Mistakes:

• Using change_column to rename instead of change type
• Removing and adding columns loses data
• Confusing rename_table with rename_column