No such polymorphic_identity is defined
moggie
Hope you are well. In my example I have two objects 'HttpTest' and
'SmtpTest' that both inherit from their super-class called 'Test'.
Due to the nature of the relationship, 'Test' will never actually be
stored in the database so I've not mapped that. However, a mixture of
'HttpTest' and 'SmtpTest' objects are both to be stored in the
hosttest_table, thus I created a mapping for them like this:
httptest_mapper = mapper(HttpTest, hosttest_table, \
polymorphic_on=hosttest_table.c.type,
polymorphic_identity='http')
smtptest_mapper = mapper(SmtpTest, hosttest_table, \
polymorphic_on=hosttest_table.c.type,
polymorphic_identity='smtp')
Unfortunately when I do, I get this error message:
No such polymorphic_identity 'smtp' is defined
I was hoping SQLA would understand this and let me pack the two
objects into the same table, but for some reason it doesn't like it.
If someone could please give me some pointers on how to proceed, it
would be greatly appreciated. Thank you in advance for your time and
consideration.
Kind regards,
moggie.
Michael Bayer
the pattern you're attempting here is called "single table inheritance".
You will need to define a common base class between your two classes, but
it never need be instantiated directly. Full documentation on how to use
this approach is at
http://www.sqlalchemy.org/docs/05/mappers.html#single-table-inheritance .
>
> Kind regards,
> moggie.
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mog
Hi Michael,
Thank you very much for getting back to me. I think I realise now that
to make this work I *must* also map the super-class of the polymorphic
objects and not map just the ones I want to use. This seems to be
working for me now.
Thanks again :)