[Python] after_insert hook - Instance is not persistent within ths Session

I've got these two SQLModels:

class Bar(SQLModel, table=True):
id: int = Field(primary_key=True, default=None)
some_field: int

class Foo(SQLModel, table=True):
id: int = Field(primary_key=True, default=None)
bar_id: int = Field(foreign_key="Bar.id")
bar: Bar = Relationship()


I'm trying to set a hook on Foo to fire whenever a new item is added. Within that hook, I need access to the bar:

def hook(_mapper, _connection, target):
do_something_with(target.bar.some_field)

sqlalchemy.event.listen(Foo, "after_insert", hook)


However, target.bar is None. I tried refreshing the object:

def hook(_mapper, _connection, target):
session = sqlalchemy.orm.object_session(target)
assert session is not None
session.refresh(target)
do_something_with(target.bar.some_field)


but I got an error:

Instance '<Foo at 0xdeadbeef>' is not persistent within this session
​

How do I get a fully populated Foo when one is created?

Continue reading...