Can someone explain when exactly implicit transactions occur?

[ersinakinci]

I understand the "unit of work" model, or at least I think I do, but the docs aren't clear on how different API methods map onto transactions.

For example, if I want to create new DB records, I can do

const myRecord = new MyEntity(...);
em.persist(myRecord);

const anotherRecord = new AnotherEntity(...);
em.persist(anotherRecord);

em.flush();  // Writes both records to the db within a transaction

But I can also do

await em.create(MyEntity, { ... });
await em.create(AnotherEntity, { ... });

em.flush();  // Is this even necessary? Does it do anything?

It's unclear to me that the second approach results in records being created within an implicit transaction.

Likewise, using .find seems to instantly retrieve records--are those statements wrapped inside of a transaction?

[B4nan]

em.create is sync method, it does the same as your previous example - create entity instance (via constructor) and persist it. its the flush call that does things (and you need to await that one instead of the create call). that's described here

Likewise, using .find seems to instantly retrieve records--are those statements wrapped inside of a transaction?

Nope, you need explicit transaction demarcation for that.

https://mikro-orm.io/docs/transactions#approach-2-explicitly

With that approach, you no longer need the explicit flush, as it is called automatically after the callback (as well as via explicit em.commit()).

[ersinakinci]

Thank you @B4nan! Also didn't realize that em.create was sync, so all these awaits I'm using are unnecessary.

What happens if you do a .find with an em instance that's waiting to be flushed? Does it do the query "out of band," so to speak, and allow the other persisted transactions to be flushed later on as usual? Or does it flush right away?

[B4nan]

There is a queue for the flush calls (or better say uow.commit calls), they need to got sequentially as they work with the state. But you can fork and flush forks in parallel if that's what you want.

edit: but the find call will be resolved right ahead, that's not blocking on its own

[ersinakinci]

Great, thanks!