Hello,
thanks again for the answer. Perhaps my case was not clear enough. To make
it a bit more explicit, I updated the example accordingly.
from sqlalchemy import event
from sqlalchemy import *
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker, backref, relationship
Base = declarative_base()
#########################################################################################
# MODEL
#########################################################################################
class House(Base):
__tablename__ = 'house'
id = Column(Integer, primary_key=True)
rooms = relationship("Room",
backref=backref("house", lazy="joined"),
cascade='all, delete-orphan')
class Room(Base):
__tablename__ = 'room'
id = Column(Integer, primary_key=True)
house_id = Column(Integer, ForeignKey('house.id'))
beds = relationship("Bed",
backref=backref("room", lazy="joined"),
cascade='all, delete-orphan')
class Bed(Base):
__tablename__ = 'bed'
id = Column(Integer, primary_key=True)
room_id = Column(Integer, ForeignKey('room.id'))
#########################################################################################
# CONFIG
#########################################################################################
def setup():
engine = create_engine("sqlite://", echo=True)
Base.metadata.bind = engine
Base.metadata.create_all(engine)
SessionFactory = sessionmaker(
bind=engine
)
event.listen(SessionFactory, 'deleted_to_detached',
listener_bed_has_been_removed)
return SessionFactory
def listener_bed_has_been_removed(session, instance):
if type(instance) is not Bed:
return
# so, in this example, this function should be called 3 times.
# The first time it is called, I get no problems, I can access
instance.room.house_id the call proceeds
# The second bed is a problem, I get the error
# "sqlalchemy.orm.exc.DetachedInstanceError: Parent instance <Bed at
0x7f24fe14bb70> is not bound to a Session; lazy load operation of attribute
'room' cannot proceed"
# SO, my question is: is there ANY way to keep these references to parents
in this function?
bed_id = instance.id
house_id = instance.room.house_id
print("execute a service call to external service here bed_id {}, house_id
{}".format(bed_id, house_id))
if __name__ == "__main__":
session_factory = setup()
session = session_factory()
session.add(House(id=1))
session.add(Room(id=1, house_id=1))
session.add(Bed(id=1, room_id=1))
session.add(Bed(id=2, room_id=1))
session.add(Bed(id=3, room_id=1))
session.commit()
room = session.query(Room).get(1)
session.delete(room)
session.commit()
session.close()
So, for this example, I am looking for a solution to keep the references to
the 'house' from the 'bed' model after flush (any solution would be good).
Is there perhaps a way to dynamically set a property in 'bed' -model (e.g
house_id), once 'room'-backref has been loaded ?
On Tuesday, August 22, 2017 at 5:36:39 PM UTC+2, Mike Bayer wrote:
>
> you would need to illustrate an MCVE of what's happening. objects
> don't "lose track" of their related objects unless yes, you deleted
> them, in which case you should not expect that they would be there.
> when the object is expired, it will go to reload them, and they'll be
> gone.
>
> On Tue, Aug 22, 2017 at 11:05 AM, cecemel <ruiz.fe...@gmail.com
> <javascript:>> wrote:
> > @update:
> >
> > calling the flush doen't seem to make any difference. At some point, the
> > object looses track of it's grandparents
> >
> >
> > On Tuesday, August 22, 2017 at 3:57:23 PM UTC+2, cecemel wrote:
> >>
> >> Hi,
> >>
> >> so, I'm currently facing this issue, where I would like to do some
> calls
> >> to an external service, when my object has been deleted within a flush.
> >> For this operation to occur, I need the id from my model object, but
> also
> >> the id from the parent of the parent object model. There are cases,
> where I
> >> am unable to access them (I guess, depending of the order of the
> execution)
> >> and I am unsure on what to do next.
> >>
> >> So if you're willing to give me some advice, would be awesome.
> >>
> >> Here is a dummy model:
> >>
> >> from sqlalchemy import event
> >>
> >> from sqlalchemy import *
> >> from sqlalchemy.ext.declarative import declarative_base
> >> from sqlalchemy.orm import sessionmaker, backref, relationship
> >>
> >> Base = declarative_base()
> >>
> >>
> >>
> >>
> #########################################################################################
>
>
> >> # MODEL
> >>
> >>
> #########################################################################################
>
>
> >> class House(Base):
> >> __tablename__ = 'house'
> >> id = Column(Integer, primary_key=True)
> >> rooms = relationship("Room",
> >> backref=backref("house", lazy="joined"),
> >> cascade='all, delete-orphan')
> >>
> >>
> >> class Room(Base):
> >> __tablename__ = 'room'
> >> id = Column(Integer, primary_key=True)
> >> house_id = Column(Integer, ForeignKey('house.id'))
> >> beds = relationship("Bed",
> >> backref=backref("room", lazy="joined"),
> >> cascade='all, delete-orphan')
> >>
> >>
> >> class Bed(Base):
> >> __tablename__ = 'bed'
> >> id = Column(Integer, primary_key=True)
> >> room_id = Column(Integer, ForeignKey('room.id'))
> >>
> >>
> >>
> >>
> #########################################################################################
>
>
> >> # CONFIG
> >>
> >>
> #########################################################################################
>
>
> >> def setup():
> >> engine = create_engine("sqlite:///foo.db", echo=True)
> >>
> >> Base.metadata.bind = engine
> >> Base.metadata.create_all(engine)
> >>
> >> SessionFactory = sessionmaker(
> >> bind=engine
> >> )
> >>
> >> event.listen(SessionFactory, 'deleted_to_detached',
> >> listener_bed_has_been_removed)
> >>
> >> return SessionFactory
> >>
> >>
> >> def listener_bed_has_been_removed(session, instance):
> >> if type(instance) is not Bed:
> >> return
> >>
> >> bed_id = instance.id
> >> house_id = instance.room.house.id # this is NOT ALWAYS there.
> >> Depending on the order of the execution I guess
> >>
> >> print("execute the service call to external service here bed_id {},
> >> house_id {}".format(bed_id, house_id))
> >>
> >>
> >>
> >>
> >> So my question(s):
> >>
> >> Is there a clean way to always acces this parent's parent attribute?
> >>
> >> If not, would be starting a new session and query it from the event
> >> handler be an option? (is it not dangerous, because it seems to work)
> >>
> >> Additional quirk, I am working within a transaction manager
> (pyramid_tm)
> >> and ZopeTransactionExtension()
> >>
> >> Thanks!
> >>
> >>
> >> More information about the system:
> >>
> >> SQLAlchemy 1.1.13
> >>
> >> Python 3.5
> >>
> >> Postgres 9.6
> >
> > --
> > 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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>
--
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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