You can get a reference to the MongoClient instance that is created by Meteor’s mongo package and then call the transaction functions (e.g., startSession and commitTransaction) using this client:
I don’t have a complete example at the moment but you could check out this test case, which updates two documents atomically using the transaction APIs.
Quick question, on the test example you posted the updates are done using rawCollection, so right now that is the only way to work with transactions? meaning they wont work using meteor/mongo?