get() query does not load joinedloaded relationship when reloading expired object

sample.py

from sqlalchemy import Column, BigInteger, ForeignKey, create_engine
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()
Session = sessionmaker()

class Foo(Base):
    __tablename__ = 'foo'

    id = Column(BigInteger, primary_key=True, autoincrement=True)
    bars = relationship('Bar', backref='foo', lazy='joined')

class Bar(Base):
    __tablename__ = 'bar'

    foo_id = Column(BigInteger, ForeignKey('foo.id'), primary_key=True)
    bar_id = Column(BigInteger, primary_key=True, autoincrement=False)

class Hello(Base):
    __tablename__ = 'hello'

    id = Column(BigInteger, primary_key=True, autoincrement=True)


def main(sqlalchemy_dabatase_uri):
    engine = create_engine(sqlalchemy_dabatase_uri, echo=True)
    Base.metadata.create_all(bind=engine)
    Session.configure(bind=engine)
    session = Session()

    try:
        foo = Foo()
        session.add(foo)
        session.commit()

        hello = Hello()
        session.add(hello)
        session.commit()

        print('----------')
        session.query(Foo).get(1)
        print('----------')
    except:
        pass

    Base.metadata.drop_all(bind=engine)

if __name__ == '__main__':
    main('mysql+pymysql://local:Password!2@localhost:3306/test_utf8mb4?charset=utf8mb4')

sample2.py

from sqlalchemy import Column, BigInteger, ForeignKey, create_engine
from sqlalchemy.orm import relationship, sessionmaker
from sqlalchemy.ext.declarative import declarative_base

Base = declarative_base()
Session = sessionmaker()

class Foo(Base):
    __tablename__ = 'foo'

    id = Column(BigInteger, primary_key=True, autoincrement=True)
    bars = relationship('Bar', backref='foo', lazy='joined')

class Bar(Base):
    __tablename__ = 'bar'

    foo_id = Column(BigInteger, ForeignKey('foo.id'), primary_key=True)
    bar_id = Column(BigInteger, primary_key=True, autoincrement=False)


def main(sqlalchemy_dabatase_uri):
    engine = create_engine(sqlalchemy_dabatase_uri, echo=True)
    Base.metadata.create_all(bind=engine)
    Session.configure(bind=engine)
    session = Session()

    try:
        foo = Foo()
        foo.bars.append(Bar(bar_id=1))
        foo.bars.append(Bar(bar_id=2))
        session.add(foo)
        session.commit()

        print('----------')
        foo.bars
        print('----------')
    except:
        pass

    Base.metadata.drop_all(bind=engine)

if __name__ == '__main__':
    main('mysql+pymysql://local:Password!2@localhost:3306/test_utf8mb4?charset=utf8mb4')

sample2.py run result:

2016-08-29 13:52:51,991 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2016-08-29 13:52:51,992 INFO sqlalchemy.engine.base.Engine SELECT foo.id AS foo_id 
FROM foo 
WHERE foo.id = %(param_1)s
2016-08-29 13:52:51,992 INFO sqlalchemy.engine.base.Engine {'param_1': 2}
2016-08-29 13:52:51,993 INFO sqlalchemy.engine.base.Engine SELECT bar.foo_id AS bar_foo_id, bar.bar_id AS bar_bar_id 
FROM bar 
WHERE %(param_1)s = bar.foo_id
2016-08-29 13:52:51,993 INFO sqlalchemy.engine.base.Engine {'param_1': 2}