SQLAlchemy Self-referencing Table Cannot Have 0 as Primary Index?

I encountered a very strange problem with SQLAlchemy. I have a model that is self-referencing (adjacency list relationship). I simply copied the model (Node) from the SQLAlchemy tutorial. Here is the model's code:

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
    )

I reproduced the problem within my controllers, but I also ran this test (after fully loading my environment of course):

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()

The above code works just fine (the changes are successfully committed and reflected in the DB).

The problem arises when I change the parent node's id to 0 (and the child's parent_id to 0 accordingly). Then, I get the following exception:

......
File "C:\Python26\Lib\site-packages\MySQLdb\connections.py", line 36, in defaulterrorhandler
raise errorclass, error开发者_如何学运维value
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')

Surprisingly, changing this value (the node's id and the child's parent_id) to anything but 0 (-5, 1 and 150) makes the error go away.

Am I missing something obvious? Is it not possible to assign 0 to a self-referencing Integer id column?

Thanks!

MySQL uses generated IDs when you explicitly pass NULL or 0. Pass autoincrement=False as argument to Column to dissable AUTO_INCREMENT.