here's a pastebin of it:
http://pastebin.com/z8XWsv2e
On Aug 16, 2011, at 7:42 PM, Michael Bayer wrote:
>
> On Aug 16, 2011, at 5:37 PM, Mike Gilligan wrote:
>
>> I have a single table that looks similar to the following:
>>
>> class Equipment(Base):
>> type = Column(CHAR(1), primary_key=True)
>> sub_type = Column(CHAR(1), primary_key=True)
>> code = Column(CHAR(5), primary_key=True)
>>
>>
>> For historical purposes, I cannot modify this table. I would like to setup
>> multi-level inheritance similar to this, however it does not work:
>>
>> class Equipment(Base):
>> type = Column(CHAR(1), primary_key=True)
>> sub_type = Column(CHAR(1), primary_key=True)
>> code = Column(CHAR(5), primary_key=True)
>> __mapper_args__ = {'polymorphic_on': type}
>>
>>
>> class Vehicle(Equipment):
>> __mapper_args__ = {'polymorphic_identity': 'V', 'polymorphic_on':
>> sub_type}
>>
>>
>> class Bus(Vehicle)
>> __mapper_args__ = {'polymorphic_identity': 'B'}
>>
>>
>> class Rail(Vehicle)
>> __mapper_args__ = {'polymorphic_identity': 'R'}
>>
>>
>> I can concatenate the multiple column values into a single discriminator
>> column_property but then I do not have an easy way to retrieve all vehicles.
>> Any ideas?
>
> The inheritance querying does handle multi-level inheritance so if your
> discriminator was on a concatenation of both things would work just fine,
> i.e. if you queried for Vehicle, etc. Each object's "polymorphic_identity"
> would need to include the concatenated value, of course.
>
> Unfortunately we're just beginning to support inheritance discriminators on a
> column_property(), and you need to use a very specific approach to make this
> work right now. There's some tickets in trac to allow this functionality out
> of the box. Attached is an example script which exercises the above
> mapping - it uses declarative to minimize the impact of the workaround.
>
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>
>
>
>
>
>>
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> <multi_discrim.py>
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