groups_table = sql.Table(
    'groups', meta.metadata,
    sql.Column('id', sql.Integer, primary_key=True),
    sql.Column('name', sql.Unicode(50)),
)

class Group(MyOrm): pass

#-----

# Association table
group2group_table = sql.Table(
    'group2group', meta.metadata,
    sql.Column('parent_id', sql.Integer, sql.ForeignKey('groups.id')),
    sql.Column('child_id', sql.Integer, sql.ForeignKey('groups.id')),
)

#-----

# Mapper
orm.mapper(Group, groups_table, properties={
    'children':orm.relation(
        Group,
        primaryjoin=(groups_table.c.id==group2group_table.c.parent_id),
        secondaryjoin=(groups_table.c.id==group2group_table.c.child_id),
        secondary=group2group_table,
        foreign_keys=[groups_table.c.id, groups_table.c.id],
        ),
    }
)