Migration is a way to manage database schema changes over time.

With migrations, you can update your db structure without manually running SQL scripts. TypeORM generates these migrations for you, and i can apply them automatically to the db.

Key Features:

1. Version Control: Each migration file is timestamped, allowing you to track schema change overtime.
2. Rollback changes.
3. Consistency

Create → run → revert

Example workflow with TypeORM Migrations

1. Setting Up Migrations: First, configure TypeORM to use migrations by specifying migration-related options in your ormconfig.js file.

   // ormconfig.js (or ormconfig.json)
   module.exports = {
     type: 'postgres',
     host: 'localhost',
     port: 5432,
     username: 'test',
     password: 'test',
     database: 'test_db',
     entities: ['src/**/*.entity.ts'],
     migrations: ['src/migration/*.ts'],  // Location where migrations will be stored
     cli: {
       migrationsDir: 'src/migration',   // Directory for generating migration files
     },
     synchronize: false,  // Disable synchronize when using migrations
   };
   
   

   The Synchronize: false option is important because it ensure Type ORM won’t automatically sync your entities with the db schema allowing migrations to take control

2. Generating a Migration: WHen you make changes to your entities, you can generate a migration that reflects those changes. TypeORM will automatically detect the differences between the current db schema and your entities and create the necessary SQL commands to update the schema

   pnpm typeorm migration:generate -n MigrationName
   

   • MigraitonName is a descriptive name for migration, like AddAgeColumnToUsers or CreateOrdersTable
   • This command will generate a new migration file in the src/migration/ directory with SQL commands needed to apply the schema changes.

   import { MigrationInterface, QueryRunner } from 'typeorm';
   
   export class AddAgeColumnToUsers1640548236143 implements MigrationInterface {
     public async up(queryRunner: QueryRunner): Promise<void> {
       await queryRunner.query(`ALTER TABLE "users" ADD "age" integer`);
     }
   
     public async down(queryRunner: QueryRunner): Promise<void> {
       await queryRunner.query(`ALTER TABLE "users" DROP COLUMN "age"`);
     }
   }
   
   

   • up() method: defines the SQL commands that will be executed when the migration is applied.
   • down() method: Defines how to revert the migration, undoing the schema changes ( here removing the age column)

Running the migration

pnpm typeorm migration:run

Reverting

pnpm typeorm migration:revert

When to Use TypeORM Migrations:

• After Modifying Entities: When you modify your entity classes (adding/removing columns, changing relationships, etc.), you should generate a migration to reflect those changes in the database.
• Collaborating on a Team: Migrations ensure that all team members have the same database schema, regardless of when they pull the latest changes.
• Production Environments: In production, database schema changes must be carefully controlled. Migrations allow you to deploy these changes in a consistent and reliable way.

Key Commands for TypeORM Migrations:

1. Generate a migration:

   bashCopy code
   pnpm typeorm migration:generate -n MigrationName