Implement control over migration transaction

[peter-episource]

Is your feature request related to a problem? Please describe.
I have a multiple tenant database structure where I would like to sync up migrations across tenant databases. For example, if I have two databases, A and B, this requires that if database A's migrations succeed, but database B's migrations fail, both database A and B's transactions rollback.

Describe the solution you'd like
In my scenario, I would have a MikroORM instance for each tenant database where I'd like to control commiting and rollback of the transactions.

The migration methods accept a transaction as a option:

migrator.up({ transaction: trx });

A way to pull a transaction from the orm. Currently I do a hacky workaround by doing:

const connection = orm.em.getDriver().getConnection();
// @ts-ignore
const trx = await connection.client.transaction();

This way, I can wait for all the migrations in the different databases to finish and commit the transactions only if all migration instances succeed.

[B4nan]

In v4 there is em.getConnection().begin() that will give you the tx context, so the latter should be fine already.

[B4nan]

await orm.em.transactional(async em => {
  await migrator.up({ transaction: em.getTransactionContext() });
});