That is not going to work.
let me explain:
Polymorphic inheritance uses tables and discriminators and some logic somewhere
in the system. (As far as I can tell both SA and databases can do the job, i
have to look into that)
For each object there will be a table, including the "base" object.
simple:
class x(BAse)
Id
discriminator
some_column_x
class y(x)
id ….
some_column_y
class z(x)
id …
some_column_y
will result in the following table structure (assume the name of the object is
the name of the table and discriminator:
x:
Id
discriminator
some_column_x
y:
Id
some_column_y
z
Id
some_column_z
now if you create a record in y or z two records are created, one in y or z and
one in x. Take note that the Id's will be the same and that the discriminator
in x will be y or z.
the Id are a sequence of the x table.
If you however create an x record, only one record will be created.
You are planning on changing from x to y or z later in the process.
you can do so but there is little more work to do here.
what you have to compensate for is the missing record in y or z
so if you have an x object you want to convert into an y object (Or Eployee to
Engineer)
Load the Employee Object.
create a new Engineer object
copy the "shared" values from the Employee, including Id and EXCLUDING
discriminator!!!
now you have converted the Employee into an Engineer
to make it persistant, remeber that we miss the record in Engineer. Saving
Engineer by Session.add(EngineerInstance) would do the trick.
You should test if it is needed to Session.delete(EmployeeInstance), and
Session.commit() first. Session.add(Engineer) might very well overwrite but It
might throw up a duplicate key error.
This will help you out I think.
Martijn
On Jan 18, 2012, at 19:08 , Thierry wrote:
> Hi
>
> I'm trying to mimick the example from the documentation (employee/
> manager/engineer)
>
> I've been able to add employees, and engineers, and all works as
> expected
>
> now my next step would be, creating an 'Employee' object, and some
> time later, decide that he's in fact an engineer.
>
> so my first attempt would read something like
>
> person = session.query(Employee).filter(..)
> -> retrieve the instance
> person.type='engineer'
> session.commit()
>
> and I would have expected at that point, when retrieving again
> person = session.query(Engineer).filter(..)
>
> to now have an instance of Engineer
> but that's not what I am seeing, I still get an instance of Employee
>
> I haven't found this issue addressed in the doc, sorry if I missed it
> What should be the right way to achieve this ? Am I missing something
> obvious here ?
>
> Many thanks in advance
>
> ---
> I can provide a complete working code of course, I just would like to
> make a sanity check before I go through the trouble of extracting the
> relevant part
>
>
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