On Tue, Apr 5, 2022, at 1:02 PM, Thorsten von Stein wrote:
> I'm currently trying to get my applications ready for SQLAlchemy 2.0. A
> change that has forces code changes in numerous places is the removal of the
> automatic addition of new instances to the session upon establishing a
> relationship with an object already in the session. In the explanation of the
> change this is described "not generally a desirable behavior". However, it is
> a behavior on which I currently completely rely for persisting new instances.
> Since, in my system, new instances are always connected to a parent object, I
> never had to add any object explicitly to a session, and so session objects
> are currently basically absent from the application code.
>
> Here are my questions:
> - Will there be a way to turn back on this behavior in 2.0? I would be
> thrilled if there was, but I fear the answer is negative.
the whole behavior is done using event handlers which is public API. so you
could set up the attributeevents.append() [1] and attributeevents.set() [2]
event handlers to add the local side to the session:
from sqlalchemy import Column
from sqlalchemy import create_engine
from sqlalchemy import event
from sqlalchemy import ForeignKey
from sqlalchemy import Integer
from sqlalchemy import String
from sqlalchemy.orm import declarative_base
from sqlalchemy.orm import object_session
from sqlalchemy.orm import relationship
from sqlalchemy.orm import Session
from sqlalchemy.orm import configure_mappers
Base = declarative_base()
class A(Base):
__tablename__ = "a"
id = Column(Integer, primary_key=True)
data = Column(String)
bs = relationship("B", backref="a")
class B(Base):
__tablename__ = "b"
id = Column(Integer, primary_key=True)
a_id = Column(ForeignKey("a.id"))
data = Column(String)
configure_mappers()
@event.listens_for(B.a, "set")
def set_(target, newvalue, oldvalue, initiator):
sess = object_session(newvalue)
if target not in sess:
sess.add(target)
e = create_engine("sqlite://", echo=True)
Base.metadata.create_all(e)
s = Session(e)
a1 = A()
s.add(a1)
s.commit()
b1 = B(a=a1)
assert b1 in s.new
[1]
https://docs.sqlalchemy.org/en/14/orm/events.html?highlight=attributeevents#sqlalchemy.orm.AttributeEvents.append
[2]
https://docs.sqlalchemy.org/en/14/orm/events.html?highlight=attributeevents#sqlalchemy.orm.AttributeEvents.set
there's an example of how to add new attribute events automatically at
https://docs.sqlalchemy.org/en/14/_modules/examples/custom_attributes/listen_for_events.html
, though you'd want to alter it to test each new attribute for being an
"object" based attribute.
> - How would you recommend dealing with the situation: Either passing sessions
> as additional parameters to any function or method that creates new objects,
> or, calling the session maker within each such function? In particular, is
> there any particular downside to the latter?
For a given chain of methods that are working in the same transaction I would
have a Session that's passed around to all of them, likely via some contextual
object, so that they can all ensure the object is added to the persistence
context as needed. calling a sessionmaker inside of each method implies
you're building a new session/transaction inside of each method which means
each method would need to commit its own transaction, did you mean the
scoped_session()? scoped_session is an option as well if you are already using
that, though I favor an explicit session/context being passed around as this
allows functions and methods to explicitly state that they need to take place
within this particular kind of context.
>
>
>
>
>
> --
> SQLAlchemy -
> The Python SQL Toolkit and Object Relational Mapper
>
> http://www.sqlalchemy.org/
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SQLAlchemy -
The Python SQL Toolkit and Object Relational Mapper
http://www.sqlalchemy.org/
To post example code, please provide an MCVE: Minimal, Complete, and Verifiable
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