Migrations for schema changes in Express

Introduction

Migrations help you change your database structure safely over time. They keep track of changes so your app and database stay in sync.

When you add a new table or column to your database.

When you rename or remove a column without losing data.

When you update the data type of a column.

When you want to share database changes with your team.

When deploying updates to production without breaking the app.

Syntax

Express

npx sequelize-cli migration:generate --name migration_name

// Inside the generated migration file:
module.exports = {
  up: async (queryInterface, Sequelize) => {
    // commands to apply changes
  },
  down: async (queryInterface, Sequelize) => {
    // commands to undo changes
  }
};

The up function applies the changes.

The down function reverts them if needed.

Examples

Adds an optional age column to the Users table and removes it on rollback.

Express

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

Creates a new Posts table with id and title columns, and drops it on rollback.

Express

module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.createTable('Posts', {
      id: {
        type: Sequelize.INTEGER,
        primaryKey: true,
        autoIncrement: true
      },
      title: {
        type: Sequelize.STRING,
        allowNull: false
      }
    });
  },
  down: async (queryInterface) => {
    await queryInterface.dropTable('Posts');
  }
};

Sample Program

This migration adds a new email column to the Users table. The column is required and unique. If you undo the migration, the column is removed.

Express

/* migration file: 20240601-add-email-to-users.js */
module.exports = {
  up: async (queryInterface, Sequelize) => {
    await queryInterface.addColumn('Users', 'email', {
      type: Sequelize.STRING,
      allowNull: false,
      unique: true
    });
  },
  down: async (queryInterface) => {
    await queryInterface.removeColumn('Users', 'email');
  }
};

Important Notes

Always write the down method to safely undo changes.

Test migrations on a development database before applying to production.

Use descriptive names for migration files to remember their purpose.

Summary

Migrations help manage database changes step-by-step.

They have up and down functions to apply and undo changes.

Using migrations keeps your app and database structure in sync safely.

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