Why is assignment of a transient parent object does not automatically add this parent object into the session?

[Bao Niu]

Why is assignment of a transient parent object does not automatically add this parent object into the session? But Parent.append(PersistedChild) will do the job nicely?

Here is my code:

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

Base = declarative_base()

class Department(Base):
    __tablename__ = 'department'
    id = Column(Integer, primary_key=True)
    name = Column(String)

class Employee(Base):
    __tablename__ = 'employee'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    department_id = Column(Integer, ForeignKey('department.id'))
    department = relationship(Department, backref=backref('employees', uselist=True))

engine = create_engine('sqlite:///')
session = sessionmaker()
session.configure(bind=engine)
Base.metadata.create_all(engine)
s = session()
john = Employee(name='john')
it_department = Department(name='IT')
print(john in s) # should be False
print(it_department in s) # should be False
s.add(john)
print(john in s) # should be True
print(it_department in s) # should be False
john.departments = it_department # here I expect it_department(the Parent Object) would be automatically added to s in a cascade way, because of the backref
print(john in s) # should be True
print(it_department in s) # should be True, however this is not true.

[Simon King]

I don’t know if this is the problem, but you appear to have a typo:

john.departments = it_department

...but the relationship is called “department”, not “departments”

HTH,

Simon

[Bao Niu]

I guess what I must do immediately is to get myself a pair of new glasses...Thank you Simon!

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