I’ve just run across the same problem, and after testing I have found that NONE of these answers are sufficient.
Currently, or as of sqlalchemy .6+, there is a very simple solution (I don’t know if this exists in prior version, though I imagine it does):
session.refresh()
So, your code would look something like this:
f = Foo(bar=x)
session.add(f)
session.flush()
# At this point, the object f has been pushed to the DB,
# and has been automatically assigned a unique primary key id
f.id
# is None
session.refresh(f)
# refresh updates given object in the session with its state in the DB
# (and can also only refresh certain attributes - search for documentation)
f.id
# is the automatically assigned primary key ID given in the database.
That’s how to do it.