Deletion of object with relationship(lazy='raise')

[Marat Sharafutdinov]

from sqlalchemy import Column, ForeignKey, Integer, create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker

Base = declarative_base()

class Parent(Base):
    __tablename__ = 'parents'
    id = Column(Integer, primary_key=True)

class Child(Base):
    __tablename__ = 'children'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey(Parent.id))
    parent = relationship(Parent, lazy='raise', passive_deletes=True)

engine = create_engine('sqlite:///:memory:', echo=True)
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
session = Session()

# Add
parent = Parent()
session.add(parent)
session.flush()
child = Child(parent_id=parent.id)
session.add(child)
session.commit()

# Delete
child = session.query(Child).first()
session.delete(child)
session.commit()

Now I'm getting the following warning:

/.venv/lib/python3.8/site-packages/sqlalchemy/orm/relationships.py:2021: SAWarning: On Child.parent, 'passive_deletes' is normally configured on one-to-many, one-to-one, many-to-many relationships only.

If I change `parent` relationship as follows:

parent = relationship(Parent, lazy='raise', backref=backref('children', passive_deletes=True))

then I would get the following exception:

sqlalchemy.exc.InvalidRequestError: 'Child.parent' is not available due to lazy='raise'

How should it be made properly?

[Mike Bayer]

lazy="raise" on a many to one is problematic because a lot of many-to-one operations involve pulling up the object from the identity map, and that's it.    since people are usually only trying to guard against SQL being emitted, use the raise_on_sql option instead:

parent = relationship(Parent, lazy='raise_on_sql')

thanks for the perfect test case BTW makes this very easy for me

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[Mike Bayer]

that said, I'm looking at this as a potential bug because it should be able to leave this attribute alone, not really sure how this should be handled.

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[Mike Bayer]

I can confirm this is a bug as I've found precedent for this not having to raise, new issue is

https://github.com/sqlalchemy/sqlalchemy/issues/4997

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