I've spent time unsuccessfully trying to fix some problems
with a many-to-many relationship and lazy joins.
Here's a simplified repro:
#!/usr/bin/env python
# -*- coding: utf-8 -*-
import random
from sqlalchemy import create_engine, ForeignKey
from sqlalchemy import Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import relationship, sessionmaker, joinedload
engine = create_engine('sqlite:///appts.db', echo=True)
Base = declarative_base()
#LAZYJOIN = 'dynamic'
LAZYJOIN = 'joined'
class Appointment(Base):
__tablename__ = 'appointments'
id = Column(Integer, primary_key=True, autoincrement=True,
nullable=False)
subject = Column(String)
persons = relationship(
'AppointmentPerson',
cascade='delete, save-update, merge, expunge',
lazy=LAZYJOIN)
# rel1 = relationship('Foo')
# rel2 = relationship('Bar')
class Person(Base):
__tablename__ = 'persons'
id = Column(Integer, primary_key=True, autoincrement=True,
nullable=False)
name = Column(String)
class AppointmentPerson(Base):
"""augmented association table between Appointment and Person"""
__tablename__ = 'appointment_persons'
appointment_id = Column(Integer, ForeignKey(Appointment.id,
ondelete='CASCADE'), nullable=False, primary_key=True)
person_id = Column(Integer, ForeignKey(Person.id), nullable=False,
primary_key=True)
person = relationship(Person)
# Other columns omitted that are necessary for the real use case
Base.metadata.create_all(engine)
Session = sessionmaker(bind=engine)
def create_data(num_appts=20, num_people=20):
random.seed(12345)
session = Session()
persons = []
for i in range(num_people):
person = Person(name="Person_%03d" % (i+1))
session.add(person)
persons.append(person)
session.flush()
for i in range(num_appts):
appt = Appointment(subject="Appt_%03d" % (i+1))
session.add(appt)
session.flush()
random.shuffle(persons)
attendee_count = random.randrange(4) + 1
for k in range(1, attendee_count):
p = persons[k]
print i, k, p.name
ap = AppointmentPerson(appointment_id=appt.id,
person_id=p.id)
appt.persons.append(ap)
session.add(ap)
session.commit()
def query_all_appointments(session):
query = session.query(Appointment)
if LAZYJOIN == 'joined':
query = query.options(joinedload(Appointment.persons)
.joinedload(AppointmentPerson.person))
return query.all()
def show_data():
print "show_data"
session = Session()
appts = query_all_appointments(session)
print len(appts)
for a in appts:
people = ", ".join([ap.person.name for ap in a.persons])
x = "{0}: {1}".format(a.subject, people)
def update_appointment_people(appt_id):
session = Session()
appt = session.query(Appointment).filter(Appointment.id ==
appt_id).one()
appt.persons.delete()
session.commit()
if __name__ == '__main__':
create_data()
show_data()
update_appointment_people(7)
The code originally used
Appointment.persons = relationship(
'AppointmentPerson',
cascade='delete, save-update, merge, expunge',
lazy='dynamic')
where everything worked, but accessing the persons on every appointment
triggered a separate query for AppointmentPersons, followed by
queries for each Person.
Changing the relationship to `lazy='joined'` and using joinedload
query = (session.query(Appointment).
options(joinedload(Appointment.persons).
joinedload(AppointmentPerson.person)))
return query.all()
reduces this to one SQL query:
SELECT appointments.id AS appointments_id,
appointments.subject AS appointments_subject,
persons_1.id AS persons_1_id,
persons_1.name AS persons_1_name,
appointment_persons_1.appointment_id AS
appointment_persons_1_appointment_id,
appointment_persons_1.person_id AS appointment_persons_1_person_id
FROM appointments
LEFT OUTER JOIN
appointment_persons AS appointment_persons_1
ON appointments.id = appointment_persons_1.appointment_id
LEFT OUTER JOIN persons AS persons_1
ON persons_1.id = appointment_persons_1.person_id
which is great.
Aside: In the original code, we're using MySQL, not SQLite.
On my MacBook, I have been unable to reproduce the pathological select
behavior
with MySQL 5.6, whereas it repros consistently on Amazon's Elastic Beanstalk
with MySQL 5.5 at RDS. (SQLAlchemy 0.9.2, Python 2.7 in both cases.)
I found this baffling.
However, with `lazy='joined'` and using joinedload,
I no longer know how to delete the AppointmentPersons
associated with an Appointment. `appt.persons.delete()` used to work;
now I get `AttributeError: 'InstrumentedList' object has no attribute
'delete'`
I've tried a variety of different approaches, all unsuccessfully.
Other approaches end up with IntegrityErrors and other things I can't
remember now.
The real reason why I want to delete AppointmentPersons is
because I have the following pattern in my REST APIs.
create:
appt = Appointment()
session.add(appt)
session.flush()
# appt.id is now valid
update_model_from_request(appt, request.json_body)
get_by_id:
return session.query(Appointment).filter(Appointment.id ==
request_id).one()
update_by_id:
appt = flush_model(request_id)
update_model_from_request(appt, request.json_body)
delete_by_id
appt = flush_model(request_id)
session.delete(appt)
flush_model:
appt = get_by_id(request_id)
# Purge various relationships
appt.rel1.delete()
appt.rel2.delete()
appt.people.delete() # <--
return appt
update_model_from_request(appt, json_body):
# set the fields in an empty appt from json_body
for p in json_body['people']:
person = session.query(Person).filter(Person.id ==
p['id']).first() or Person()
person.name = p['name']
session.add(person)
session.flush()
ap = AppointmentPerson(appointment_id=appt.id,
person_id=person.id)
session.add(ap)
# update rel1 and rel2
This isn't especially efficent and leads to some churn in Persons
and AppointmentPersons, but it's easy to write.
Is there a better way to accomplish this? I could, for the
update_by_id case, partition into (added, deleted, unchanged)
but that's somewhat tedious to compute.
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