SQLAlchemy issue with joinedload and correlate_except on a column_property

joinedload-correlate.py

from sqlalchemy import create_engine, select, and_, func
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Boolean, Column, Integer, String, ForeignKey
from sqlalchemy.orm import sessionmaker, column_property, relationship, joinedload, deferred, undefer, load_only

engine = create_engine('sqlite:///:memory:', echo=True)
Session = sessionmaker(bind=engine)
session = Session()
Base = declarative_base()


class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    name = Column(String)
    fullname = Column(String)
    password = Column(String)
    group_id = Column(Integer, ForeignKey('groups.id'))
    group = relationship('Group')

    def __repr__(self):
       return "<User(id=%d, name='%s', fullname='%s', password='%s')>" % (
           self.id, self.name, self.fullname, self.password)


class Worker(User):
    __tablename__ = 'workers'

    id = Column(Integer, ForeignKey('users.id', ondelete='CASCADE'), primary_key=True)
    nino = Column(String)
    employed = Column(Boolean)
    group = relationship('WorkersGroup', backref='workers')


class Group(Base):
    __tablename__ = 'groups'

    id = Column(Integer, primary_key=True)
    name = Column(String)

    def __repr__(self):
       return "<Group(id=%d, name='%s')>" % (self.id, self.name)


class WorkersGroup(Group):
    __tablename__ = 'workers_groups'

    id = Column(Integer, ForeignKey('groups.id', ondelete='CASCADE'), primary_key=True)

    def __repr__(self):
       return "<WorkersGroup(id=%d, name='%s', count='%r')>" % (
           self.id, self.name, self.employee_count)

WorkersGroup.employee_count = column_property(
    select([func.count(Worker.id)]).where(
        and_(
            Worker.id == User.id,
            Worker.group_id == WorkersGroup.id,
            Worker.nino.isnot(None)
        )
    ).correlate_except(Worker, User)
)



Base.metadata.create_all(engine)

workers = WorkersGroup(name='workers')
session.add(workers)

contractors = WorkersGroup(name='contractors')
session.add(contractors)
session.flush()

ed_user = User(name='ed', fullname='Ed Jones', password='edspassword')
session.add(ed_user)

ed1_user = Worker(name='ed1', fullname='Ed1 Jones', group_id=workers.id, nino='ABC123')
session.add(ed1_user)

ed2_user = Worker(name='ed2', fullname='Ed2 Jones', group_id=workers.id)
session.add(ed2_user)

ed3_user = Worker(name='ed3', fullname='Ed3 Jones', group_id=contractors.id, nino='CBA321')
session.add(ed3_user)

ed4_user = Worker(name='ed4', fullname='Ed4 Jones', group_id=contractors.id, nino='CBA321')
session.add(ed4_user)

ed5_user = Worker(name='ed5', fullname='Ed5 Jones', group_id=contractors.id, nino='CBA321')
session.add(ed5_user)
session.flush()

# Depending on which of the following blocks happen first, the result is either 1 or 3

# Block 1
w = session.query(Worker).filter(Worker.id == 4).one()
print(w)
print(w.group.employee_count)

# Block 2
w = session.query(Worker).options(joinedload(Worker.group)).filter(Worker.id == 4).one()
print(w)
print(w.group.employee_count)