aaa

users = Table('users', meta,    Column('id', Integer, Sequence('user_id_seq'), primary_key=True),    Column('username', String(100), unique=True))groups = Table('groups', meta,    Column('id', Integer, Sequence('group_id_seq'), primary_key=True),    Column('groupname', String(100), unique=True),    Column('owner_user_id', Integer, ForeignKey('users.id'), nullable=False),    Column('parent_group_id', Integer, ForeignKey('groups.id')))members = Table('members', meta,    Column('user_id', Integer, ForeignKey('users.id'), nullable=False),    Column('group_id', Integer, ForeignKey('groups.id'), nullable=False),    UniqueConstraint('user_id', 'group_id', name='members_udx_1'))