Migrations in NestJS

Introduction

Migrations help you change your database step-by-step safely. They keep track of what changes were made so your data stays organized.

When you add a new table to your database.

When you change a column type or name.

When you remove a column or table.

When you want to share database changes with your team.

When you deploy updates to your app's database.

Syntax

NestJS

npx typeorm migration:create -n MigrationName

// Then edit the generated migration file:
export class MigrationName implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    // code to apply changes
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    // code to undo changes
  }
}

Use up to add or change database structure.

Use down to undo those changes if needed.

Examples

This command creates a new migration file named AddUsersTable.

NestJS

npx typeorm migration:create -n AddUsersTable

This migration adds a users table in up and removes it in down.

NestJS

export class AddUsersTable implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`CREATE TABLE users (id INT PRIMARY KEY, name VARCHAR(100))`);
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`DROP TABLE users`);
  }
}

This command runs all pending migrations to update the database.

NestJS

npx typeorm migration:run

This command reverts the last run migration to undo changes.

NestJS

npx typeorm migration:revert

Sample Program

This migration creates a products table with id, name, and price columns. Running up adds the table. Running down removes it.

NestJS

import { MigrationInterface, QueryRunner } from 'typeorm';

export class AddProductsTable1680000000000 implements MigrationInterface {
  public async up(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`
      CREATE TABLE products (
        id INT PRIMARY KEY AUTO_INCREMENT,
        name VARCHAR(255) NOT NULL,
        price DECIMAL(10,2) NOT NULL
      )
    `);
  }

  public async down(queryRunner: QueryRunner): Promise<void> {
    await queryRunner.query(`DROP TABLE products`);
  }
}

// To run this migration, use:
// npx typeorm migration:run

// To revert it, use:
// npx typeorm migration:revert

Important Notes

Always write the down method to safely undo changes.

Run migrations in your development and production environments to keep databases in sync.

Use descriptive names for migrations to remember what each one does.

Summary

Migrations help you change your database step-by-step.

Use up to apply changes and down to undo them.

Run migrations with commands like migration:run and migration:revert.

Practice

1. What is the main purpose of migrations in a NestJS project using TypeORM?

easy

A. To handle HTTP requests and responses

B. To safely update the database schema step-by-step

C. To write business logic for the application

D. To manage user authentication

Solution

1. Step 1: Understand what migrations do

   Migrations help change the database structure safely without losing data.

2. Step 2: Identify the correct purpose in NestJS context

   They are not for business logic or HTTP handling but for database schema updates.

3. Final Answer:

   To safely update the database schema step-by-step -> Option B

4. Quick Check:

   Migrations = safe database updates [OK]

Hint: Migrations change database structure safely [OK]

Common Mistakes:

• Confusing migrations with business logic
• Thinking migrations handle HTTP requests
• Assuming migrations manage authentication

2. Which command correctly runs all pending migrations in a NestJS project using TypeORM?

easy

A. npx typeorm migration:create

B. npx typeorm migration:revert

C. npx typeorm migration:generate

D. npx typeorm migration:run

Solution

1. Step 1: Recall commands for migrations

   migration:run applies pending migrations to the database.

2. Step 2: Differentiate from other commands

   migration:create creates a new migration file, migration:generate generates migration from schema changes, and migration:revert undoes the last migration.

3. Final Answer:

   npx typeorm migration:run -> Option D

4. Quick Check:

   Run migrations = migration:run [OK]

Hint: Run migrations with migration:run command [OK]

Common Mistakes:

• Using migration:create to run migrations
• Confusing migration:generate with migration:run
• Using migration:revert to apply migrations

3. Given this migration class, what will happen when up() is run?

export class AddAgeColumn1678901234567 {
  async up(queryRunner) {
    await queryRunner.query(`ALTER TABLE users ADD COLUMN age int`);
  }
  async down(queryRunner) {
    await queryRunner.query(`ALTER TABLE users DROP COLUMN age`);
  }
}

medium

A. Creates a new table called 'age'

B. Removes the 'age' column from the 'users' table

C. Adds a new integer column 'age' to the 'users' table

D. Throws a syntax error because of missing semicolons

Solution

1. Step 1: Analyze the up() method

   The up() method runs SQL to add a column named 'age' of type integer to the 'users' table.

2. Step 2: Understand the effect of the query

   This modifies the existing 'users' table by adding the new column; it does not remove columns or create new tables.

3. Final Answer:

   Adds a new integer column 'age' to the 'users' table -> Option C

4. Quick Check:

   up() adds column 'age' [OK]

Hint: up() applies changes like adding columns [OK]

Common Mistakes:

• Confusing up() with down() method
• Thinking it creates a new table
• Assuming syntax error due to semicolons

4. You wrote this migration but get an error when running npx typeorm migration:run. What is the likely problem?

export class RemoveEmailColumn1678901234568 {
  async up(queryRunner) {
    await queryRunner.query(`ALTER TABLE users DROP email`);
  }
  async down(queryRunner) {
    await queryRunner.query(`ALTER TABLE users ADD email varchar(255)`);
  }
}

medium

A. The SQL syntax for dropping a column is incorrect

B. The migration class name is invalid

C. Missing async keyword in down() method

D. The migration file is not saved

Solution

1. Step 1: Check the SQL in up() method

   The query ALTER TABLE users DROP email is incorrect SQL syntax for dropping a column.

2. Step 2: Correct SQL syntax for dropping a column

   The correct syntax is ALTER TABLE users DROP COLUMN email. Missing 'COLUMN' causes the error.

3. Final Answer:

   The SQL syntax for dropping a column is incorrect -> Option A

4. Quick Check:

   DROP COLUMN keyword required in SQL [OK]

Hint: Use DROP COLUMN to remove columns in SQL [OK]

Common Mistakes:

• Omitting COLUMN keyword in DROP statement
• Assuming class name causes error
• Forgetting async keyword (not the cause here)

5. You want to create a migration that renames a column 'username' to 'user_name' in the 'accounts' table and can be reverted. Which up() and down() methods are correct?

hard

A. async up(queryRunner) {
await queryRunner.query(`ALTER TABLE accounts RENAME COLUMN username TO user_name`);
}
async down(queryRunner) {
await queryRunner.query(`ALTER TABLE accounts RENAME COLUMN user_name TO username`);
}

B. async up(queryRunner) {
await queryRunner.query(`ALTER TABLE accounts CHANGE username user_name varchar(255)`);
}
async down(queryRunner) {
await queryRunner.query(`ALTER TABLE accounts CHANGE user_name username varchar(255)`);
}

C. async up(queryRunner) {
await queryRunner.query(`ALTER TABLE accounts DROP COLUMN username`);
await queryRunner.query(`ALTER TABLE accounts ADD COLUMN user_name varchar(255)`);
}
async down(queryRunner) {
await queryRunner.query(`ALTER TABLE accounts DROP COLUMN user_name`);
await queryRunner.query(`ALTER TABLE accounts ADD COLUMN username varchar(255)`);
}

D. async up(queryRunner) {
await queryRunner.query(`RENAME COLUMN username TO user_name`);
}
async down(queryRunner) {
await queryRunner.query(`RENAME COLUMN user_name TO username`);
}

Solution

1. Step 1: Identify correct SQL syntax for renaming columns

   The standard SQL syntax supported by many databases is ALTER TABLE table_name RENAME COLUMN old_name TO new_name.

2. Step 2: Check each option's SQL correctness and revert logic

   async up(queryRunner) {
     await queryRunner.query(`ALTER TABLE accounts RENAME COLUMN username TO user_name`);
   }
   async down(queryRunner) {
     await queryRunner.query(`ALTER TABLE accounts RENAME COLUMN user_name TO username`);
   }

   uses correct syntax for both up() and down() to rename and revert the column name.

   async up(queryRunner) {
     await queryRunner.query(`ALTER TABLE accounts CHANGE username user_name varchar(255)`);
   }
   async down(queryRunner) {
     await queryRunner.query(`ALTER TABLE accounts CHANGE user_name username varchar(255)`);
   }

   uses MySQL-specific CHANGE syntax which may not be portable.

   async up(queryRunner) {
     await queryRunner.query(`ALTER TABLE accounts DROP COLUMN username`);
     await queryRunner.query(`ALTER TABLE accounts ADD COLUMN user_name varchar(255)`);
   }
   async down(queryRunner) {
     await queryRunner.query(`ALTER TABLE accounts DROP COLUMN user_name`);
     await queryRunner.query(`ALTER TABLE accounts ADD COLUMN username varchar(255)`);
   }

   drops and adds columns, losing data.

   async up(queryRunner) {
     await queryRunner.query(`RENAME COLUMN username TO user_name`);
   }
   async down(queryRunner) {
     await queryRunner.query(`RENAME COLUMN user_name TO username`);
   }

   misses the table name in the query, causing error.

3. Final Answer:

   ALTER TABLE accounts RENAME COLUMN username TO user_name -> Option A

4. Quick Check:

   Rename column with ALTER TABLE RENAME COLUMN [OK]

Hint: Use ALTER TABLE RENAME COLUMN for renaming columns [OK]

Common Mistakes:

• Dropping and adding columns instead of renaming
• Omitting table name in ALTER TABLE
• Using database-specific syntax without checking compatibility