Is multi-level polymorphism possible in SQLAlchemy?
- by Jace
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 IntegrityError: (IntegrityError) entities.entity_type may not be NULL. This error goes away if I remove the Image model and the polymorphic_on key in File.
What gives?
(Edited: the exception raised was wrong.)
