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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