5.4.10. Write Migration

In this chapter, you'll learn how to create a migration and write it manually.

What is a Migration?#

A migration is a class created in a TypeScript or JavaScript file under a module's migrations directory. It has two methods:

• The up method reflects changes on the database.
• The down method reverts the changes made in the up method.

How to Write a Migration?#

The Medusa CLI tool provides a db:generate command to generate a migration for the specified modules' data models.

Alternatively, you can manually create a migration file under the migrations directory of your module.

For example:

src/modules/hello/migrations/Migration20240429.ts
1import { Migration } from "@mikro-orm/migrations"2
3export class Migration20240702105919 extends Migration {4
5  async up(): Promise<void> {6    this.addSql("create table if not exists \"my_custom\" (\"id\" text not null, \"name\" text not null, \"created_at\" timestamptz not null default now(), \"updated_at\" timestamptz not null default now(), \"deleted_at\" timestamptz null, constraint \"my_custom_pkey\" primary key (\"id\"));")7  }8
9  async down(): Promise<void> {10    this.addSql("drop table if exists \"my_custom\" cascade;")11  }12
13}

The migration's file name should be of the format Migration{YEAR}{MONTH}{DAY}.ts. The migration class in the file extends the Migration class imported from @mikro-orm/migrations.

In the up and down method of the migration class, you use the addSql method provided by MikroORM's Migration class to run PostgreSQL syntax.

In the example above, the up method creates the table my_custom, and the down method drops the table if the migration is reverted.

Run the Migration#

To run your migration, run the following command:

Terminal
❯npx medusa db:migrate

This reflects the changes in the database as implemented in the migration's up method.