That makes sense, thank you, Simon!
Regarding the events: you suggest to use a before_flush() to examine
session.dirty whenever a session.query() executes?
Also, is there a way to get the list of objects that have been flushed, or
should I track them myself whenever a before_flush() event occurs?
Jens
On Thursday, November 16, 2017 at 7:49:54 PM UTC+10, Simon King wrote:
>
> On Thu, Nov 16, 2017 at 7:45 AM, <jens.t...@gmail.com <javascript:>>
> wrote:
> > Hello,
> >
> > I've been exploring some of the session functionality that handles
> object
> > states, and I'm quite confused. Here is what I see:
> >
> >>>> engine = engine_from_config({'sqlalchemy.url': 'mysql+pymysql://…'})
> >>>> session_factory = sessionmaker(bind=engine) # No autoflush
> >>>> session = session_factory()
> >>>> # Now query a table to get an object.
> >>>> p = session.query(Table).filter(Table.id == '0f4…ed6').one_or_none()
> >>>> p.name
> > "Some Name"
> >>>> p.name = "Other Name"
> >>>> session.dirty
> > IdentitySet([<Table object at 0x10bb50d68>])
> >>>> session.is_modified(p)
> > True
> >>>> session._is_clean()
> > False
> >
> > This all makes sense. If I now create a new query, then the above change
> > seems to be gone, but isn't?
> >
> >>>> p2 = session.query(Table).filter(Table.id ==
> '384…a05c').one_or_none()
> >>>> session.dirty
> > IdentitySet([])
> >>>> session.is_modified(p)
> > False # p is not modified according to this.
> >>>> session._is_clean()
> > True
> >>>> p.name
> > "Other Name" # p still has the modified name.
> >
> > The new query seems to set the session to "clean", but the object p
> still
> > contains its change. I can't quite find documentation for the behavior.
> What
> > am I missing?
> >
> > What I would like to do is: in one session, select a few objects
> (multiple
> > queries), inspect, perhaps modify them. Then I'd like to query if there
> were
> > any modifications/deletes and if so choose to commit or rollback.
> Initially
> > I thought to use the Session.dirty/deleted/new properties, but then the
> > above showed.
> >
> > If I was to set autoflush, how could I inspect the flushed code buffer?
> >
> > Thanks!
> > Jens
>
> The results you are seeing are due to autoflush, which is on by
> default. When you run your second query, the session flushes any
> pending changes to the database before running the query. After the
> flush, the session is considered clean. The methods you were using
> (dirty, is_modified etc.) indicate whether the session contains
> changes that haven't been flushed to the database. They don't tell you
> if the database has received changes that haven't yet been committed.
>
> There are various ways to do what you want, depending on how your code
> is structured. One possibility is to use a Session event such as
> before_flush to set a flag saying that there are uncomitted changes in
> the database.
>
> http://docs.sqlalchemy.org/en/latest/orm/session_events.html
>
> Hope that helps,
>
> Simon
>
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