http://www.sqlalchemy.org/docs/session.html?highlight=expung...The following is more of a screed than a personal response to your post, so please don't take it personally.
This is something I run into quite a bit more than you might expect. Partly it is because I used to spend a lot of time in #sqlalchemy, #pylons, #pocoo, #python, and #postgresql on freenode (I'm 'ltbarcly' on freenode, feel free to msg me HNers!). For some reason it is common to believe that you can sit down and write applications that use an ORM and relational database without knowing both the ORM and relational databases.
If you are going to use relational databases, you should be prepared to become an expert in SQL, Database drivers, unicode (and why utf-8 or any other encoding isn't the same as unicode, in the same way that jpg or gif isn't the same as image data), indexing, data types, normalization tradeoffs, transaction isolation levels, locking semantics, ACID tradeoffs, how to read and understand query planner output, and much more. This is a lot to know, but if it were a college class it would be less than a semester long.
If you are going to use an ORM you have to know everything you would need when you use a relational database, and in addition how and when the ORM performs queries, how it maps to the database, what queries it will produce and when for any given action or attribute access, how db transactions map into the orm operations, how to perform common SQL operations via the orm (rather than loading more data than you need and filtering or joining via the application).