How to check whether SQLAlchemy session is dirty or not

python, sqlalchemy

Solution

you're looking for a net count of actual flushes that have proceeded for the whole span of the session's transaction; while there are some clues to whether or not this has happened (called the "snapshot"), this structure is just to help with rollbacks and isn't strong referencing. The most direct route to this would be to track "after_flush" events, since this event only emits if flush were called and also that the flush found state to flush:

from sqlalchemy import event
import weakref
transactions_with_flushes = weakref.WeakSet()

@event.listens_for(Session, "after_flush")
def log_transaction(session, flush_context):
    for trans in session.transaction._iterate_parents():
        transactions_with_flushes.add(trans)

def session_has_pending_commit(session):
    return session.transaction in transactions_with_flushes

edit: here's an updated version that's a lot simpler:

from sqlalchemy import event

@event.listens_for(Session, "after_flush")
def log_transaction(session, flush_context):
    session.info['has_flushed'] = True

def session_has_pending_commit(session):
    return session.info.get('has_flushed', False)

Problem

I have a SQLAlchemy `Session` object and would like to know whether it is dirty or not. The exact question what I would like to (metaphorically) ask the `Session` is: "If at this point I issue a `commit()` or a `rollback()`, the effect on the database is the same or not?". The rationale is this: I want to ask the user wether he wants or not to confirm the changes. But if there are no changes, I would like not to ask anything. Of course I may monitor myself all the operations that I perform on the `Session` and decide whether there were modifications or not, but because of the structure of my program this would require some quite involved changes. If SQLAlchemy already offered this opportunity, I'd be glad to take advantage of it. Thanks everybody.

Original source