Is is possible to set the polymorphic_on attribute on an object that is
not directly tied to a db table, but has access to the db attribute via
delegation?
I have a generic Product class that processes an XML to obtain its
generic attributes (uuid, type, etc). Afterwards, the product is
encapsulated within one of several proxy objects that perform additional
processing based on the enclosed product type. Each of the various
proxies are derived from a BaseProxy and multiple types can use
the same proxy. Due to this, the BaseProxy has a poly_type to specify
which of the proxies was used for additional processing.
Base = declarative_base()
class Product(Base):
__tablename__ = 'products'
id = Column(Integer, Sequence('id_seq'), primary_key=True)
type = Column(String(16))
class BaseProxy(Base):
__tablename__ = 'products'
__table_args__ = {'extend_existing': True}
poly_type = Column(String(16))
__mapper_args__ = {'polymorphic_on': poly_type}
product = relationship(Product, uselist=False)
def __init__(self, product):
self.product = product
def __getattr__(self, attrib):
return getattr(self.product, attrib)
class HardwareProduct(BaseProxy, Base):
__tablename__ = 'hardware'
__mapper_args__ = {'polymorphic_identity': 'hardware'}
id = Column(Integer, ForeignKey('products.id'), primary_key=True)
serial = Column(String(16))
class SoftwareProduct(BaseProxy, Base):
__tablename__ = 'software'
__mapper_args__ = {'polymorphic_identity': 'software'}
id = Column(Integer, ForeignKey('products.id'), primary_key=True)
product_key = Column(String(16))
However:
>>> p = Product()
sqlalchemy.exc.ArgumentError: Could not determine join condition between
parent/child tables on relationship ProductProxy.product. Specify a
'primaryjoin' expression. If 'secondary' is present, 'secondaryjoin' is
needed as well.
>>>
In anticipation of one response, the reason that I didnt have Hardware
and SoftwareProduct inherit directly from Product was because the
product's type wasn't known until after product creation and, due to the
number of products created, I didn't want to waste the time recreating
the underlying product again and again. However, I'm willing to change
it if there is a better approach. Product creation looks something like:
p = Product('product.xml')
p = select_proxy_based_on_type(p.type)(p)
I realize this is a drawn out question, but any help is appreciated.
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