detached.py

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


Base = declarative_base()
eng = create_engine('postgresql+psycopg2:///asdf', echo=True)


class Parent(Base):
    __tablename__ = 'parent'
    id = Column(Integer, primary_key=True)


class Child(Base):
    __tablename__ = 'child'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey(Parent.id))

    parent = relationship(
        Parent,
        lazy='joined',
        backref=backref(
            'children',
            lazy='joined')
    )

Base.metadata.drop_all(eng)
Base.metadata.create_all(eng)

s = Session(bind=eng)
s.add(Parent(id=1))
s.add(Child(id=1, parent_id=1))

c = s.query(Child).all()
s.close()

print c[0].parent  # works just fine because parent was joined by default using `lazy='joined` in rel
print c[0].parent.children  # throws