QGIS Expression Field Calculator Function Editor doesn't recognise Geometry methods

Question

It appears as though the function editor on the Field Calculator is not recognizing the Geometry() method on the feature.

Example code below, always returns -1 into the field I populate from the Field Calculator. The features have valid geometry. The source table is standalone and not a join. When I also do a print(geometry()) call, it returns <QgsGeometry: null> The weird thing is that on the Field Calculator preview, it shows the correct value.

The issue is similar to this issue:

Expression not saving in QGIS Field Calculator?

However the tables I am working with have no joins. I have tested on both 3.18 and 3.22 versions with a blank project. No joins/relationships.

from qgis.utils import qgsfunction
from qgis.core import *
from qgis.gui import *
    
@qgsfunction(args='auto', group='Custom')
    
def test_geom(feature, parent):
    
    area_calc = feature.geometry().area()
        
    return area_calc

Answer 1

Seriously, I've had this problem all day, scouring google, 10 minutes after posting the question - of course, I stumble across the answer on the git forums.

The reason why this doesn't work is explained in detail here https://github.com/qgis/QGIS/issues/41695

In short, the Preview doesn't use the same code as the actual function method, and in order to get it to work, you need to declare usesgeometry=True in the function declaration.

So this works.

from qgis.utils import qgsfunction
from qgis.core import *
from qgis.gui import *
    
@qgsfunction(args='auto', group='Custom', usesgeometry=True)
    
def test_geom(feature, parent):
    
    area_calc = feature.geometry().area()
        
    return area_calc