Database folder and migrations in Ruby on Rails - Cheat Sheet & Quick Revision

Recall & Review

beginner

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

The db/migrate folder stores migration files that describe changes to the database schema, like creating or modifying tables. These files help Rails track and apply database changes in order.

beginner

What command do you run to create a new migration in Rails?

You run rails generate migration MigrationName to create a new migration file with a timestamp in the db/migrate folder.

intermediate

How do migrations help keep the database schema consistent across different environments?

Migrations record each change as a file with a timestamp. Running rails db:migrate applies all pending migrations in order, ensuring all environments have the same database structure.

intermediate

What is the role of the schema.rb file in Rails?

The schema.rb file is a snapshot of the current database structure. It is updated after migrations run and helps Rails recreate the database schema quickly without running all migrations.

intermediate

What happens if you try to run rails db:migrate but some migrations have already been applied?

Rails checks which migrations have been applied by looking at the schema_migrations table. It only runs new migrations that haven't been applied yet, so it avoids repeating changes.

Where are Rails migration files stored?

A lib/tasks

B app/models

C db/migrate

D config

Migration files live in the db/migrate folder to organize database schema changes.

What command creates a new migration file?

A rails new migration

B rails generate migration MigrationName

C rails db:create migration

D rails generate model MigrationName

Use rails generate migration followed by the name to create a migration file.

What does rails db:migrate do?

A Runs all pending migrations to update the database schema

B Deletes the database

C Creates a new Rails project

D Generates a new model

It applies all migrations that have not yet been run to update the database.

What file shows the current database schema snapshot?

A seeds.rb

B routes.rb

C database.yml

D schema.rb

schema.rb reflects the current structure of the database after migrations.

How does Rails know which migrations have already run?

A By checking the schema_migrations table in the database

B By reading the migration filenames

C By checking the config folder

D By scanning the app/models folder

Rails tracks applied migrations in the schema_migrations table to avoid rerunning them.

Explain the purpose of the db/migrate folder and how migrations work in Rails.

Think about how Rails tracks and applies database changes step-by-step.

Describe the role of the schema.rb file and how it relates to migrations.

Consider how Rails can rebuild the database structure without running all migrations again.

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