How can I listen for the creation of a specific model and create a new one (on a different table) based on this? - postgresql

Question

I have a User model with a referral_key attribute. I'd like to create a ReferralKeyRecord upon creation of a user. I've read tons of documentation and StackExchange to no avail.
This answer uses after_insert(), but I am not trying to alter or validate the class which is being inserted; I am trying to add a new object from a completely different model—and session.add() isn't supported.
This answer is closer to what I want, but the accepted answer (ultimately) uses after_flush(), which is far too general. I don't want to listen to events thrown whenever the DB is updated somehow. I want it to fire off when a specific model is created.
And something like the following...
#event.listens_for(User, 'after_flush')
def create_referral_record(mapper, connection, target):
session.add(ReferralRecord(key=instances.referral_key))
session.commit()
... results in No such event 'after_flush' for target '<class 'models.User'>. I've looked through SQLAlchemy's documentation (the core events and the ORM events) and see no events that indicate a specific model has been created. The closest thing in Mapper events is the after_insert method, and the closest thing in Session events is the after_flush() method. I imagine this is a pretty common thing to need to do, and would thus be surprised if there wasn't an easy event to listen to. I assume it'd be something like:
#event.listens_for(User, 'on_creation')
def create_referral_record(session, instance):
record = ReferralRecord(key=instance.referral_key)
session.add(record)
session.commit()
Does anyone know better than I?

Answer 1

Or why not create the Referral inside the User constructor?
from sqlalchemy.orm import Session, relationship, Mapper
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, ForeignKey, create_engine, event
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
def __init__(self):
self.referral = Referral()
id = Column(Integer(), primary_key=True)
referral = relationship('Referral', uselist=False)
class Referral(Base):
__tablename__ = 'referral'
id = Column(Integer(), primary_key=True)
user_id = Column(Integer(), ForeignKey('user.id'), nullable=False)
engine = create_engine('sqlite:///:memory:')
Base.metadata.create_all(engine)
session = Session(bind=engine)
session.add(User())
session.commit()
print(session.query(User).all())
print(session.query(Referral).all())

Answer 2

You can use the after_flush session event, and inside the event handler you can access the session's new objects (using session.new).
Example:
from sqlalchemy.orm import Session, relationship, Mapper
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer, ForeignKey, create_engine, event
Base = declarative_base()
class User(Base):
__tablename__ = 'user'
id = Column(Integer(), primary_key=True)
class Referral(Base):
__tablename__ = 'referral'
id = Column(Integer(), primary_key=True)
user_id = Column(Integer(), ForeignKey('user.id'), nullable=False)
engine = create_engine('sqlite:///:memory:')
Base.metadata.create_all(engine)
session = Session(bind=engine)
#event.listens_for(session, 'after_flush')
def session_after_flush(session, flush_context):
for obj in session.new:
if isinstance(obj, User):
session.add(Referral(user_id=obj.id))
session.add(User())
session.commit()
print(session.query(User).all())
print(session.query(Referral).all())
Running this outputs:
[<__main__.User object at 0x00000203ABDF5400>]
[<__main__.Referral object at 0x00000203ABDF5710>]