🔴 Transactions are not used now!

[Olegt0rr]

Bug

In the _save() method transaction's session is not passed to update_one() request.

odmantic/odmantic/engine.py

Lines 315 to 320 in 541d411

	await collection.update_one(
	{"_id": getattr(instance, instance.__primary_field__)},
	{"$set": doc},
	upsert=True,
	bypass_document_validation=True,
	)

It means that transaction is NOT USED NOW!

How to fix?

It's not enough to just pass the session to request.
In MongoDB transactions are not available for Standalone.
This simple way Standalone users will get exceptions for every save() call 😢

So we need to define the users of Standalone version and not start a transaction for them.
Example:

if self.client._server_property('server_type') == SERVER_TYPE.Standalone:
    await self._save(instance, None, **kwargs)
else:
    async with await self.client.start_session() as s:
        async with s.start_transaction():
            await self._save(instance, s, **kwargs)

P.S.: I don't know how to properly check ServerType without protected _server_property method :(

[art049]

Nice catch @Olegt0rr 🤓

In MongoDB transactions are not available for Standalone.

Do you have some references about this ?
The only transaction requirements I found were those https://docs.mongodb.com/manual/core/write-operations-atomicity/
Edit: Actually, it's stated that this would only work on replicaSets

[art049]

To find out if we're connected to a replicaset or not, i found another way, using the db.serverStatus command:

server_status = await self.database.command({'serverStatus'  :1})
is_standalone = "repl" not in server_status

I tried to use the _server_property you describe but I couldn't find it on my local client.
It would be probably faster to use it as it would avoid to send an extra request to the database.

[Olegt0rr]

Do you have some references about this ?

https://docs.mongodb.com/manual/core/transactions-production-consideration/#feature-compatibility

Replica Set (4.0+) and Sharded Cluster (4.2+) support transactions.
Standalone is not listed here.
Also you can run an experiment: pass session to request and your PR tester will answer with error 😢

---

i found another way, using the db.serverStatus command

This way requires server admin role:
not authorized on admin to execute command

For self.client._server_property('server_type') - readWrite role is enough.
I've found it in pymongo similar solution:
https://github.com/mongodb/mongo-python-driver/blob/6b0123594aeb6d4fa618568a28cab3344c5422f0/pymongo/mongo_client.py#L1033-L1039

Also added request to MongoDB jira: https://jira.mongodb.org/browse/PYTHON-2476

[art049]

This way requires server admin role:
not authorized on admin to execute command

Yes 😭, I totally forgot this initial constraint.

Thanks for creating the request in Jira !
I just saw an anwser there:

Bernie Hackett added a comment - Dec 25 2020 03:30:26 PM GMT+0000
The easiest way to do what you want, with no API changes, is to call the ismaster command. If the msg field is set to "isdbgrid" you're connected to a mongos. If the setname field is set you're connected to a replica set memeber. Otherwise you're connected to a standalone.

This looks like a very good solution. We will have to check the version as well to make sure sharded cluster are 4.2+.
To add these commands in the Engine initializer, we might have to gather the event loop held by the motor client or maybe access the underlying pymongo client (I'm not sure it's doable in a clean way for this one)

[Olegt0rr]

Seems calling self.client._server_property('server_type') still easiest way 😭

[Olegt0rr]

Need some help to finish tests coverage

[art049]

Hey, yes I will merge #91 as soon as possible, this should help you with the coverage :)

[art049]

Closed by #227