On Nov 4, 2008, at 12:24 PM, john.goodleaf wrote:
> I must be doing something wrong here. I've put a function call into
> the mapper like so:
>
> mapper(Instance, instances,
> properties={'LocalInstanceName':
> column_property(select(
> [func.fnLocalizedInstanceName(bindparam('eng'),
>
> bindparam(instances.c.InstanceID)) ]))} )
>
InstanceID is a column so you wouldn't put that inside of a bind
parameter. Also the "eng" bind parameter is probably not appropriate
inside of column_property() for general query usage, since it requires
a value at query execution time which is probably not available.
While you can specify bind parameter values with Query using the
params() method, the Query object is executed against the Instance
object in other scenarios, such as when attributes are refreshed, and
the "eng" value will not be available in that case. As an example,
what would the value of "eng" be if I were to say
"session.query(Instance).get(5)" ?
A solution which might be more what you're looking for is this:
class Instance(object):
...
def get_localized_instance_name(self, eng):
return
object_session(self).execute(select([func.fnLocalizedInstanceName(eng,
self.InstanceID)]).scalar()
that way when you have an Instance, you can call
myinstance.get_localized_instance_name(<value of eng>) to get the
desired result.
If OTOH the "eng" value can be derived from the Instance row, we can
revisit the column_property() approach, which in reality only grants a
small performance advantage in that the function can be executed at
the same time the Instance object itself is selected, as opposed to a
second SELECT when an accessor is called.
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