How set event on field in SQLAlchemy, that it will call only when changes concern db

Question

I am using SQLAlchemy, and face a problem, that I need to handle event, when one field in table is changed. Firstly, I've tried to use code from docs:

@listen_for(Advert.status, 'set')
def handle_change_status(target, value, oldvalue, initiator):
    if value != oldvalue:
         # do smth

But this event occure any time the field is changed, even if we do not modify db. For example:

advert=Advert()
advert.status = 3  # event apear

But I need that event appear only when I commit changes:

db.session.add(advert)
db.session.commit()  # I need event occure here

I think that I can handle each 'set' event and write them in some storage, and then add event on session.commit (after_commit) and in this event handle all set events. But this approach seems not to be very neat.

May be someone has any ideas, how to solve this problem?

Answer 1

So, I have solve this question that way:

Firstly, I've created init where save init value: def __init__(self): self._init_value = self.value

Also I've declared method, that do the same, when orm reconstruct object from db:

@orm.reconstructor def init_on_load(self): self.__init__()

And then I created events:

@event.listens_for(Model, 'after_update')
def compare_old_and_new_values(mapper, connection, target):
    if target._init_value != target.value:
        # do smth

@event.listens_for(Model, 'after_insert')
def compare_old_and_new_values(mapper, connection, target):
    if target._init_value != target.value:
        # do smth

`

I think this approach can have some misbehaviour, when multiple users change the same record in db, but this aproach save us from making additionals SQL queries to db.