class BaseGroup(Base):
    # ...
    parent_id = Column(Integer, ForeignKey('base_groups.id'))
    parent = relationship('BaseGroup',
        foreign_keys='BaseGroup.id',
        primaryjoin='BaseGroup.parent_id == BaseGroup.id',
        uselist=False)

    child_groups = relationship('BaseGroup',
        foreign_keys='BaseGroup.parent_id',
        primaryjoin='BaseGroup.id == BaseGroup.parent_id',
        uselist=True)

    roster = relationship("BaseGroupRoster",
        order_by="BaseGroupRoster.position",
        collection_class=ordering_list('position', count_from=1),
        lazy='joined')

    @hybrid_property
    def sub_roster(self):
        roster = self.roster
        for each in self.child_groups:
            print(each.name)
            roster += each.sub_roster

        return roster