Run once

By default (runOnce: true) each seeder is tracked in a seeders table — similar to how MikroORM tracks migrations. On the next boot — including watch-mode restarts — seeders already recorded in that table are skipped.

In combination with schema drops

When the schema (including the seeders table) is dropped on application start, all seeders run again on the next boot, regardless of runOnce.

Schema dropped on start	runOnce	What happens
Yes	true	Tracking table dropped → all seeders run (fresh schema needs fresh seeds)
Yes	false	Always runs
No	true	Tracking table persists → already-run seeders are skipped
No	false	Always runs (tracking disabled)

Dropping the schema on every start is typical in development when you want a clean slate on every restart — seeding every run is exactly what you want in that scenario. runOnce: true with a persistent schema is the right default for staging and production, where duplicate data is the concern.

Evolving seed data

Treat seeders the way you treat migrations: once a seeder has run in a persistent environment, consider it immutable. If you need more data or different data, create a new seeder class rather than editing the existing one:

@Seeder()
class UserSeeder implements SeederInterface { ... }      // already ran, leave it alone

@Seeder({ dependencies: [UserSeeder] })
class UserSeederV2 implements SeederInterface { ... }   // adds more users on next boot

This keeps the history table accurate and avoids the question of what re-running a seeder would mean for data that already exists. This applies to environments where the schema persists and runOnce: true — in development with schema drops on restart, the schema is wiped anyway.

Re-seeding

To force a seeder to run again, use seed:untrack:

npm

npx @joakimbugge/mikroorm-seeder seed:untrack UserSeeder -o ./dist/orm.js

yarn

yarn @joakimbugge/mikroorm-seeder seed:untrack UserSeeder -o ./dist/orm.js

pnpm

pnpm exec @joakimbugge/mikroorm-seeder seed:untrack UserSeeder -o ./dist/orm.js

Or delete its row directly from the seeders table:

DELETE FROM "seeders" WHERE name = 'UserSeeder';

To reset everything and re-run all seeders on next boot:

DELETE FROM "seeders";

INFO

If your factories use random data (e.g. via Faker), setting runOnce: false means you will get a completely different dataset on every restart. This can be useful in some development setups, but is usually not what you want.