"""
Hello,
I was curious about a default relationship behavior. It seems that
the
default behavior when deleting a child in a relationship is to query
the parent first. I'm not sure why. This hampers something I'm
working on because I want to only give privileges to a program to only
what it will need to do its work. I have a table that has two
parents,
that is a foreign key to two tables. One of those tables isn't needed
by the application, but it needed for other applications that use
these
same SQLAlchemy models.
The following example shows first a default parent/child relationship
with the resulting query calls. The second example shows an example
using passive_deletes=True in the relationship from child to parent,
which I understand doesn't make too much sense but causes my desired
behavior. This, also, has the resulting query calls.
Is there a better way for me to get the behavior I desire, or is this
a
bug?
Thank you for any insight.
"""
from sqlalchemy import create_engine
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, relationship
from sqlalchemy.schema import Column, ForeignKey
from sqlalchemy.types import Integer
engine = create_engine('sqlite:///')
################ Without passive_deletes=True ###############
Base = declarative_base()
class Parent(Base):
__tablename__ = 'parents'
id = Column(Integer, primary_key=True)
children = relationship('Child', lazy='dynamic')
class Child(Base):
__tablename__ = 'children'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey(Parent.id), nullable=False)
parent = relationship('Parent')
Base.metadata.create_all(engine)
session = sessionmaker(bind=engine, autocommit=True)()
parent = Parent()
child = Child(parent=parent)
session.add(parent)
session.add(child)
session.flush()
session.expunge_all()
engine.echo = True
session.delete(session.query(Child).one())
session.flush()
engine.echo = False
Base.metadata.drop_all(engine)
# Ouput
"""
2011-02-11 08:16:39,242 INFO sqlalchemy.engine.base.Engine.0x...cd90
SELECT children.id AS children_id, children.parent_id AS
children_parent_id
FROM children
2011-02-11 08:16:39,243 INFO sqlalchemy.engine.base.Engine.0x...cd90
()
2011-02-11 08:16:39,243 INFO sqlalchemy.engine.base.Engine.0x...cd90
BEGIN (implicit)
2011-02-11 08:16:39,244 INFO sqlalchemy.engine.base.Engine.0x...cd90
SELECT parents.id AS parents_id
FROM parents
WHERE parents.id = ?
2011-02-11 08:16:39,244 INFO sqlalchemy.engine.base.Engine.0x...cd90
(1,)
2011-02-11 08:16:39,244 INFO sqlalchemy.engine.base.Engine.0x...cd90
DELETE FROM children WHERE children.id = ?
2011-02-11 08:16:39,244 INFO sqlalchemy.engine.base.Engine.0x...cd90
(1,)
2011-02-11 08:16:39,245 INFO sqlalchemy.engine.base.Engine.0x...cd90
COMMIT
"""
################ With passive_deletes=True ###############
Base = declarative_base()
class Parent(Base):
__tablename__ = 'parents'
id = Column(Integer, primary_key=True)
children = relationship('Child', lazy='dynamic')
class Child(Base):
__tablename__ = 'children'
id = Column(Integer, primary_key=True)
parent_id = Column(Integer, ForeignKey(Parent.id), nullable=False)
parent = relationship('Parent', passive_deletes=True)
Base.metadata.create_all(engine)
session = sessionmaker(bind=engine, autocommit=True)()
parent = Parent()
child = Child(parent=parent)
session.add(parent)
session.add(child)
session.flush()
session.expunge_all()
engine.echo = True
session.delete(session.query(Child).one())
session.flush()
engine.echo = False
# Output
"""
/usr/lib/pymodules/python2.6/sqlalchemy/orm/properties.py:897:
SAWarning: On Child.parent, 'passive_deletes' is normally configured
on one-to-many, one-to-one, many-to-many relationships only.
self._determine_direction()
2011-02-11 08:16:39,252 INFO sqlalchemy.engine.base.Engine.0x...cd90
SELECT children.id AS children_id, children.parent_id AS
children_parent_id
FROM children
2011-02-11 08:16:39,252 INFO sqlalchemy.engine.base.Engine.0x...cd90
()
2011-02-11 08:16:39,253 INFO sqlalchemy.engine.base.Engine.0x...cd90
BEGIN (implicit)
2011-02-11 08:16:39,253 INFO sqlalchemy.engine.base.Engine.0x...cd90
DELETE FROM children WHERE children.id = ?
2011-02-11 08:16:39,253 INFO sqlalchemy.engine.base.Engine.0x...cd90
(1,)
2011-02-11 08:16:39,253 INFO sqlalchemy.engine.base.Engine.0x...cd90
COMMIT
"""
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