On Tue, Apr 5, 2022, at 3:09 PM, Thorsten von Stein wrote:
> Thanks, Mike. Yes, I am using the scoped session. Am I understanding you
> correctly that, when using the scoped session, there is no penalty when
> calling the sessionmaker repeatedly, but that you still prefer passing around
> sessions as parameters for transparency reasons?
the scoped_session doesn't call "sessionmaker" repeatedly; it calls it once per
thread, until you say scoped_session.remove(). it's a thread local registry
for a persistent Session object.
I do prefer passing sessions around as global registries tend to be forgotten
and contribute to the "implicitness" of a program.
>
> On Tuesday, April 5, 2022 at 11:58:33 AM UTC-7 Mike Bayer wrote:
>>
>>
>> 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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>>> description.
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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 Example. See http://stackoverflow.com/help/mcve for a full
> description.
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http://www.sqlalchemy.org/
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