Overwrite postgresql onupdate with sqlalchemy

Question

Original problem I'm using sqlalchemy in our project. I registered few event listeners for my model like so.

@event.listens_for(Post, 'after_update')
@event.listens_for(Post, 'after_insert')

Somewhere else in the code I update the view_counter property of my model.

post.view_counter += 1
DBSession.flush()

Now, since view_counter is not something I regard as content update, I want to supress firing of after_update event in sqlalchemy for this specific update. Is there a way to do it via ORM or is the only way to just do raw SQL update bypassing ORM entirely?

EDIT After some poking around I realized there is another problem, the property is set by Postgresql itself

date_last_updated = Column(SA_DateTime, default=DateTime.now, onupdate=DateTime.now, nullable=False, index=True)

Problem
So the question is, how to update a sqlalchemy model without triggering Postgresql update.

Ruben Helsloot · Accepted Answer · 2019-07-26 17:12:38Z

2

If you're able to make changes to the decorated function, it should be possible to use the passed arguments to determine which fields were modified, albeit through a little bit of a workaround. I'm afraid I haven't been able to test this yet, but I hope it helps!

@event.listens_for(Post, 'after_update', raw=True)
@event.listens_for(Post, 'after_insert', raw=True)
def myfunc(mapper, connection, target):
    # Values have been modified
    if target.session.is_modified(target, include_collections=False):
        # view_counter is the only modified value, the hook can be aborted
        if set(target.dict.keys()) - target.unmodified == {'view_counter'}:
            return
    <...the rest of the function...>

answered Jul 26, 2019 at 17:12

Ruben Helsloot

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3 Comments

intelis Over a year ago

Thanks, this did solve original problem, but I discovered another one, I've updated the question.

Ruben Helsloot Over a year ago

You could replace onupdate=DateTime.now with onupdate=myupdatefunc and define your own context-aware update function using docs.sqlalchemy.org/en/13/core/…

Ruben Helsloot Over a year ago

Another option would be to explicitly set the updated_at value to the same value it already has, triggering an "update" of the field should mean that the onupdate clause is not executed

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