Don't use None for the Column type (i.e., detected as the "null" type). Put
the type explicitly. This has been updated in the documentation recently
since the "None" feature can't be fully supported at this time.
On May 18, 2010, at 1:34 PM, Kiran Jonnalagadda wrote:
> Is it possible to have multi-level polymorphism in SQLAlchemy? Here's
> an example:
>
> class Entity(Base):
> __tablename__ = 'entities'
> id = Column(Integer, primary_key=True)
> created_at = Column(DateTime, default=datetime.utcnow,
> nullable=False)
> entity_type = Column(Unicode(20), nullable=False)
> __mapper_args__ = {'polymorphic_on': entity_type}
>
> class File(Entity):
> __tablename__ = 'files'
> id = Column(None, ForeignKey('entities.id'), primary_key=True)
> filepath = Column(Unicode(255), nullable=False)
> file_type = Column(Unicode(20), nullable=False)
> __mapper_args__ = {'polymorphic_identity': u'file',
> 'polymorphic_on': file_type)
>
> class Image(File):
> __mapper_args__ = {'polymorphic_identity': u'image'}
> __tablename__ = 'images'
> id = Column(None, ForeignKey('files.id'), primary_key=True)
> width = Column(Integer)
> height = Column(Integer)
>
> When I call Base.metadata.create_all(), SQLAlchemy raises the
> following error: NotImplementedError: Can't generate DDL for the null
> type. This error goes away if I remove the Image model.
>
> What gives?
>
> I sense that declaring both polymorphic_identity and polymorphic_on in
> File isn't doing the expected thing, but I'm not sure how else to do
> this.
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