Additional computed fields

[lallulli]

Hi. I would like to add a new field to my FeatureType, not defined in my model, whose values are computed at runtime by a generic Python function.

I was able to add a computed field by subclassing XsdElement and overriding the get_value method, but I have to bind it to an existing field of my model (thus overriding the value read from the database). If I name my additional field with a name that does not exist in my model, I get a FieldDoesNotExist exception.