Based on [this](https://stackoverflow.com/a/52867095/1291122) SO answer I came to know that firestore does not have collection level locking in a transaction.
In my case, I have to ensure that the username field in users collection is unique before I write to a collection.
For that, I write a transaction that does this:

1. Executes a query on users collection to check if a document exists where username=something
2. If it does exist, fail and return error from transaction
3. If it does not exist, just run the write operation for the userId I want to update/create.

Now the issue here is that if two clients simultaneously try to run this transaction, both might query the collection and since the collection is not locked, one client might insert/update a document in collection while other won't see it.

Is my assumption correct? And if yes, then how to deal with such scenarios?