Sqlalchemy backend should create a transaction if it is going to call session.commit

See
https://docs.sqlalchemy.org/en/13/orm/session_transaction.html
http://alextechrants.blogspot.com/2013/08/unit-testing-sqlalchemy-apps.html

L312 of https://github.com/klen/mixer/blob/develop/mixer/backend/sqlalchemy.py will end the session's current transaction and commit them.

This will break things that rely on nested sessions, such as the example above which use nested transaction to encapsulate tests. It seems like a good argument not to use nested transactions that way, but I think you should also try to avoid interfering with user transactions if possible.

Ie. I think you should either drop the session.commit() line, and let users interact with sessions how they want, or call session.begin_nested() before session.add, so that the commit will not effect parent transactions.

Start in mixer/backend/sqlalchemy.py around line 312 and read the linked SQLAlchemy session transaction documentation and nested-session example. Reproduce how the current session.commit() affects a caller's nested transaction, then determine which transaction behavior preserves the user's session boundaries. Done means fixture creation no longer interferes with an existing parent transaction.

Written by the indexing model from the issue text.