SQLAlchemy自引用表不能将0作为主要索引吗？

class Node(Base):
    __tablename__ = \'nodes\'
    id = Column(Integer, primary_key=True)
    parent_id = Column(Integer, ForeignKey(\'nodes.id\'))
    parent = Column(Unicode(50))

    children = relationship(\'Node\',
        cascade=\"all\", #tried to remove this
        backref=backref(\"parent\", remote_side=\'Node.id\'),
        collection_class=attribute_mapped_collection(\'data\'), #tried to remove this as well
    )

parent = Node()
parent.id = 1
parent.parent_id = None
parent.name = \'parent\'
Session.add(parent)

child = Node()
child.id = 20
child.parent_id = 1
child.name = \'child\'
Session.add(child)

Session.commit()

......
File \"C:\\Python26\\Lib\\site-packages\\MySQLdb\\connections.py\", line 36, in defaulterrorhandler
raise errorclass, errorvalue
sqlalchemy.exc.IntegrityError: (IntegrityError) (1452, \'Cannot add or update a child row: a
foreign key constraint fails (`db`.`nodes`, CONSTRAINT `nodes_ibfk_1` FOREIGN KEY
(`parent_id`) REFERENCES `nodes` (`id`))\') \'INSERT INTO nodes (id, parent_id, name) VALUES 
(%s, %s, %s)\' (20, 0, \'child\')

node