On 03/10/2017 01:57 AM, Alessandro Molina wrote:
I have been looking for  a way to know what's going to be rolled back in
SQLAlchemy so that I can know what was changed and restore other
database unrelated things to their previous state.

By http://docs.sqlalchemy.org/en/latest/orm/events.html#session-events
it looks like it's available an after_soft_rollback event, but in that
even the objects already got rolled back and so their history is gone.
In the most common scenario users do Session.flush() and then
Session.rollback(), and in that case I have before_flush that can tell
me everything that changed (than I can restore the state of related
things into after_soft_rollback which can benefit from the knowledge
that I gathered in before_flush) but if the user does a direct
Session.rollback() without a flush I couldn't find an event I could
attach to know what changed an so what is going to be rolled back.

Not sure if that can be achieved or a new event would be needed.

When an integrity error or something like that happens, often the database transaction is unusable anyway, and no further SELECT can be emitted. See http://docs.sqlalchemy.org/en/latest/faq/sessions.html#but-why-does-flush-insist-on-issuing-a-rollback for detail. An event that is added before this rollback occurs would provide an environment that can't work consistently because the database connection may or may not be usable depending on specifics. So if I add that event, then I get the endless parade of "I can't do X in the before_rollback event but only when Q, P, R exist and I'm using backend Z!".

The idiomatic way to be able to rollback an operation but still continue to work with the data is to use savepoints.

If this is truly, "unexpected error but we need to do things", perhaps you can use before_flush() to memoize the details you need for a restore inside of session.info.

An event hook can be added but it would need to be carefully considered what the specific use case for this hook is. For example I'm not sure "before_rollback()" is really what this should be, it likely should be "on flush exception" similar to how engine does it.




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