Migrations for schema changes in Express - Mini Project: Build & Apply

Migrations for schema changes

📖 Scenario: You are building a simple Express app that stores user information in a database. Over time, you need to change the database schema safely without losing data. This project will guide you through creating a migration script to update the schema.

🎯 Goal: Create a migration script using Express and a migration tool to add a new column age to the users table.

📋 What You'll Learn

Create an initial users table schema

Add a configuration variable for the new column

Write a migration script to add the new column

Complete the migration setup to run the migration

💡 Why This Matters

🌍 Real World

Migrations help update database schemas safely as apps grow and change, avoiding data loss.

💼 Career

Knowing migrations is essential for backend developers to manage database changes in production environments.

1

Create initial users table schema

Create a migration file that defines a users table with columns id (integer, primary key) and name (string). Use the exports.up function to create the table.

Express

// Your migration code here
exports.up = function(knex) {
  // Create users table
};

exports.down = function(knex) {
  // Drop users table
};

Hint

Use knex.schema.createTable inside exports.up to create the table with id and name.

2

Add configuration for new column

Create a constant called newColumn and set it to the string 'age'. This will be the name of the new column to add.

Express

exports.up = function(knex) {
  return knex.schema.createTable('users', function(table) {
    table.increments('id').primary();
    table.string('name');
  });
};

exports.down = function(knex) {
  return knex.schema.dropTable('users');
};

// Define new column name
// Your code here

Hint

Define const newColumn = 'age'; above the migration functions.

3

Write migration to add new column

Modify the exports.up function to add the new column age as an integer to the existing users table using knex.schema.table. Use the newColumn constant for the column name.

Express

const newColumn = 'age';

exports.up = function(knex) {
  // Add new column to users table
  // Your code here
};

exports.down = function(knex) {
  return knex.schema.dropTable('users');
};

Hint

Use knex.schema.table('users', ...) to add the age column as integer in exports.up. Remove it in exports.down.

4

Complete migration setup

Export the migration functions exports.up and exports.down so they can be run by the migration tool. Ensure the file exports both functions correctly.

Express

const newColumn = 'age';

// Export migration functions
// Your code here

Hint

Make sure both exports.up and exports.down are defined and exported.

Practice

1. What is the main purpose of migrations in an Express app using a database?

easy

A. To create API routes for the app

B. To handle user authentication automatically

C. To manage and apply database schema changes step-by-step

D. To serve static files like images and CSS

Solution

1. Step 1: Understand migrations role

   Migrations help track and apply changes to the database schema in a controlled way.

2. Step 2: Compare other options

   Options A, B, and C relate to other app features, not schema changes.

3. Final Answer:

   To manage and apply database schema changes step-by-step -> Option C

4. Quick Check:

   Migrations = Manage schema changes [OK]

Hint: Migrations = step-by-step database structure updates [OK]

Common Mistakes:

• Confusing migrations with routing or middleware
• Thinking migrations handle user login
• Assuming migrations serve static files

2. Which of the following is the correct command to create a new migration file using a common migration tool like Sequelize CLI?

easy

A. sequelize new migration add-users-table

B. sequelize migrate new add-users-table

C. sequelize create migration add-users-table

D. sequelize migration:create --name add-users-table

Solution

1. Step 1: Recall Sequelize CLI syntax

   The correct command to create a migration is sequelize migration:create --name <name>.

2. Step 2: Check other options

   Options B, C, and D use incorrect command structures not recognized by Sequelize CLI.

3. Final Answer:

   sequelize migration:create --name add-users-table -> Option D

4. Quick Check:

   Correct CLI command = sequelize migration:create --name add-users-table [OK]

Hint: Use 'migration:create --name' to create migration files [OK]

Common Mistakes:

• Mixing up command order or keywords
• Omitting '--name' flag
• Using 'new' instead of 'create'

3. Given this migration snippet using Sequelize:

module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.addColumn('Users', 'age', { type: Sequelize.INTEGER });
  },
  down: async (queryInterface) => {
    await queryInterface.removeColumn('Users', 'age');
  }
};

What will happen when this migration is run?

medium

A. Creates a new 'Users' table with an 'age' column

B. Adds an 'age' column of type INTEGER to the 'Users' table

C. Removes the 'age' column from the 'Users' table

D. Throws an error because 'down' is missing Sequelize parameter

Solution

1. Step 1: Analyze the 'up' function

   The 'up' function adds a new column 'age' of type INTEGER to the 'Users' table.

2. Step 2: Understand the 'down' function

   The 'down' function reverses this by removing the 'age' column, but this runs only on rollback.

3. Final Answer:

   Adds an 'age' column of type INTEGER to the 'Users' table -> Option B

4. Quick Check:

   'up' adds column, 'down' removes column [OK]

Hint: 'up' applies changes, 'down' reverts them [OK]

Common Mistakes:

• Confusing 'up' and 'down' functions
• Assuming 'down' runs on migration
• Thinking it creates a new table

4. You wrote this migration to rename a column but it throws an error:

module.exports = {
  up: async (queryInterface) => {
    await queryInterface.renameColumn('Users', 'username', 'user_name');
  },
  down: async (queryInterface) => {
    await queryInterface.renameColumn('Users', 'user_name', 'username');
  }
};

What is the most likely cause of the error?

medium

A. The table name 'Users' is incorrect or does not exist

B. Missing Sequelize parameter in the functions

C. renameColumn requires the column type as a third argument

D. The migration file name is invalid

Solution

1. Step 1: Check renameColumn usage

   renameColumn needs correct table and column names; no type argument is needed.

2. Step 2: Consider error causes

   If the table 'Users' does not exist or is misnamed, renameColumn will fail.

3. Final Answer:

   The table name 'Users' is incorrect or does not exist -> Option A

4. Quick Check:

   Table name must exist for renameColumn [OK]

Hint: Check table and column names exist before renaming [OK]

Common Mistakes:

• Assuming renameColumn needs column type
• Ignoring table name case sensitivity
• Forgetting to verify table existence

5. You want to add a new column 'email' to the 'Users' table but only if it doesn't already exist, to avoid errors during migration. Which approach best handles this in an Express app migration?

hard

A. Use a raw SQL query inside the migration to check column existence before adding

B. Directly add the column without checks; rely on migration rollback if error occurs

C. Modify the model only and skip migration since model syncs automatically

D. Delete the 'Users' table first, then add the column

Solution

1. Step 1: Understand migration safety

   Adding a column without checking can cause errors if it exists already.

2. Step 2: Use raw SQL for conditional check

   Running a raw SQL query to check if the column exists before adding prevents errors.

3. Step 3: Evaluate other options

   Directly add the column without checks; rely on migration rollback if error occurs risks errors, C ignores migration purpose, D is destructive and unsafe.

4. Final Answer:

   Use a raw SQL query inside the migration to check column existence before adding -> Option A

5. Quick Check:

   Check column first with SQL to avoid errors [OK]

Hint: Check column existence with SQL before adding [OK]

Common Mistakes:

• Skipping existence check causing migration failure
• Relying only on model sync without migration
• Dropping tables unnecessarily