Tracker.py

class Tracker(Base):
    __tablename__ = 'tracker'
    
    id = Column(Integer, primary_key=True)
    state = Column(String, nullable=False)

    def __repr__(self):
        return 'Tracker<{.id}>'.format(self)


class Child(Base):
    __tablename__ = 'child'
    __table_args__ = {'sqlite_autoincrement': True}
    
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey('parent.id'))

    tracker = relationship(Tracker,
                           primaryjoin=(id == Tracker.id),
                           foreign_keys=(id,),
                           innerjoin=True,
                           uselist=False)

    def __repr__(self):
        return 'Child<{.id}>'.format(self)

    @hybrid_property
    def state(self):
        return self.tracker.state

    @state.setter
    def state(self, value):
        self.tracker.state = value

    @state.expression
    def state(cls):
        return Tracker.state


class Parent(Base):
    __tablename__ = 'parent'
    __table_args__ = {'sqlite_autoincrement': True}
    
    id = Column(Integer, primary_key=True)
    has_stopped_children1 = column_property(
        exists()
        .where(Child.parent_id == id)
        .where(Child.state == 'stop')
        .correlate_except(Tracker)
        .label('has_stopped_children'),
        )

    @hybrid_property
    def has_stopped_children2(self):
        return any(child.state == 'stop' for child in self.children)

    @has_stopped_children2.expression
    def has_stopped_children2(cls):
        return cls.children.any(Child.state == 'stop')

    children = relationship(Child, backref='parent')

    def __repr__(self):
        return 'Parent<{.id}>'.format(self)