[Sqlalchemy-users] Class Inheritance / Polymorphism

[Michael Carter]

Hello,

I am a bit confused as to how inheritance and polymorphic relationships are supposed to work. In the polymorphism.py example, the engineers and managers both have the same set of attributes: person_id, company_id, name, description. If I change the example so that engineers have a special_description but managers still have description, then it ceases to work when I try to look at c.employees.


With this approach failing I attempted to create an inheritance structure using filtered mapping (as described at http://www.objectmatter.com/vbsf/docs/maptool/ormapping.html#MappingaClassInheritanceTree ). To do this I created a Person  table that has the columns person_id, name, description, special_description, type. Then I mapped Engineer against select([Person.person_id, name, special_description]) (assigned to the var engineerSelect) and Manager against select([Person.person_id, name, description]). The next step of the plan was to create an ExtensionMapper that would create an Engineer or Person depending on the value in the type column. I'm missing some other pieces such as where I put the logic in that automatically assigns a value to the type column (a before_insert function in the mapper perhaps?) when its committed.

Anyway, this doesn't work when creating and committing objects as I get an error something like "No column description is configured on Mapper|Engineer|engineerSelect..." So should I just abandon this approach?

Finally, I have one (hopefully) trivial question. In the same polymorphic example (not that the polymorphism matters in this question) each Person object has a company_id attribute. I'm not too concerned with knowing the company_id of a Person so much as I am with being able to get an actual company object. so if I have a person john, i could say c = john.company and c would be an instance of a Company object. Likewise, I'd like to be able to create a Person with the constructor dave = Person(company=c) so that I never have to look at the company_id. Can relation(... backref='company' ...) do this?


Thanks,
Michael