On 10 Feb 2015, at 22:14, Bao Niu <niuba...@gmail.com> wrote:
>
> 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.
>
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
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