attached is a patch to the polymorph.py example that changes the name of the column in the "engineers" table from "description" to "special_description", adjusts the union'ed select for the different column name and puts a quick column translation in the MapperExtension to make the selection still work.
this example still doesnt get us into really flexible territory, such as, the engineers/managers table having large sets of different columns, which you would not want to exist on the opposite object...for stuff like that i probably need to add some more hooks to MapperExtension. it also seems like I might want to throw a more automated way to union mappers together to acheive this same effect. Michael Carter wrote: > 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 >
poly.diff
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