Hi,
First I need to let you know I do everything declarative......
To make things understandable I have chosen to use objects a bit more close to
real-life.
There is a base object Person with a one-to-many relationship to the table
addresses:
class Person(Base):
__tablename__ = "person"
Id = Column(Integer, primary_key=True)
.....
Addresses = relation('Address',primaryjoin='Address.Person_Id ==
Person.Id', cascade="all")
....
Address is a polymorphic base type:
class Address(Base):
__tablename__ = "adresses"
Id = Column(Integer,
primary_key=True)
discriminator = Column(Unicode(20))
__mapper_args__ = {'Polymorphic_on': discriminator}
....
...
for a lot of address types to be appended to Person.Addresses like:
class EmailAddress(Address):
__tablename__ = "emailaddresses"
__mapper_args__ = {'polymorphic_identity' : u'emailaddress'}
Id = Column(Integer,ForeignKey('addresses.Id'),
primary_key=True)
Value = Column(Unicode(100))
class MSNAddress(Address):
__tablename__ = "msnaddresses"
__mapper_args__ = {'polymorphic_identity' : u'msnaddress'}
Id = Column(Integer,ForeignKey('addresses.Id'),
primary_key=True)
class PostalAddress(Address):
__tablename__ = "postaladdresses"
Country = Column(Unicode(2),primary_key=True)
# Should "hold" the polymorphic Identity for
subclass like "NL","D","UK", "US"
__mapper_args__ = {'polymorphic_identity' : u'postaladdress'}
# cannot add {'Polymorphic_on' : Counrty} to
this.... see below in text what happens if I do
Id = Column(Integer,ForeignKey('addresses.Id'),
primary_key=True)
Now I want to make say a Dutch postaladdress, a german postaladdress.
The obvious way to do so is make the base class Polymorphic again but it will
"overwrite" the __mapper_args__ of the base class Address
class NLPostalAddress(PostalAddress):
__tablename__ = "nlpostaladdress"
__mapper_args__ = {'polymorphic_identity' : u'NL'} # will "overwrite"
the polymorphic Identity of PostalAddress base class
Id = Column(Integer,ForeignKey('postaladdresses.Id'),
primary_key=True)
Street = Column(Unicode(100))
HouseNumber = Column(Unicode(10))
...
...
Person.Addresses.append(NLPostalAddress(Street = "somestreet", HouseNum
......)) works but with: Address. discriminator stays empty. and
PostallAddress. Country will become 'NL'.
Which is logical (look at the comments in the "code") since the polymorphic_on
: Country makes the mapper "forget" it was polymorphic on
"Address.discriminator"
PostalAddress.Id makes the ID part work well......
Is there a way to "nest" polymorph classes in some way so the above works...
One solution might be Concrete tables but I have not been able to get that
working either? since that would involve a "polymorphic union" I might be
missing something there... but I do not really understand the documentation
(I'm still using 0.5.8, upgrading is possible)
Again this is a "fake" data model to make my point
Any suggestions?
Martijn
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