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.

[vdboor]

Hi! Sounds like a wonderful deep-dive!

Currently the fields are indeed very bound to the model. This is a based-in assumption to support query filters on all fields. You are free to loosen this with a PR, but I suspect it might be hard work.

Since Django 5 offers a GeneratedField, this would likely be the best way to implement this. It would also allow querying the data.

[Yrob]

scheduled: 135658

[jforsman]

Would be nice yes, I have done this the similar way, by subclassing XsdElement. doing with annotation does not work, as there is qset.only() calls somewhere deep in gisserver.
example what I did so you get the idea, not sure if @lallulli did it the same way:

class CentroidLocationXsdElement(XsdElement):
    def get_value(self, instance: models.Model):
        return getattr(instance, "centroid_location", None)

centroid_location is a property on a model.

[vdboor]

Thanks for the feedback! Looks like the .only() logic is the limiting factor here. Also the filter logic will expect the field to be there for now, and the .only() logic is calculated from the XsdElement.orm_path.

As intermediate solution, we do have support for GeneratedField in main (the upcoming 2.0 release). Still, Django only supports GeneratedField(db_persist=True) for Postgres now.