On Oct 30, 2008, at 5:30 PM, David Gardner wrote:
>
> This works. However although type=='file' is the only time I want/need
> to have a subclass for, I still need to know the original value of
> type.
> Is there a way in SA to basically get two copies of the column?
> Maybe I
> could return a tuple from MyRow.__getitem__ ?
>
> The best think I can think of is to create a view (PostgreSQL):
> CREATE OR REPLACE VIEW nodehierarchy_view AS SELECT *, type AS poly
> FROM nodehierarchy;
>
> and map my classes against the view, that way I have two copies of the
> type columns.
>
> To complicate things slightly more I am joining against a type table,
> who's primary key is a text field. So typically when I just want to
> know
> the name of the type I do node.type, but sometimes I need additional
> information and do a node.Type.other_info. Right now the relation is
> commented out since it isn't working.
>
> node_mapper = mapper(Node, nodehierarchy_table, extension=MyExt(),
> polymorphic_on=nodehierarchy_table.c.type,
> polymorphic_identity='node',
> properties = {
> #'Type' : relation (Type,
> primaryjoin=(type_table.c.name==nodehierarchy_table.c.type),
> uselist=False),
The way I'd want to add "polymorphic_on as a callable" would allow one
to replace the entire mechanism of the "discriminator", such that
there's not even a specific column that SQLA knows about in that
regard. This is a moderate change to the mapper internals, but the
big behavioral change is that the *population* of the column would
also become something you'd have to do on your end, and I'd also have
to figure out what its behavior with regards to querying becomes - it
would probably be several functions the user needs to implement.
Right now Im wondering if it's not easier for you to just provide a
create_instance() mapper extension that just returns the base class
for all discriminators other than "file", and you just do away with
using polymorphic_on. The fact that you are compressing all those
discriminators into one class means that you're losing a lot of the
semantics of inheritance already- you cant query for them without
explicitly saying filter(discriminator==sometype), for example.
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