Create new fields and calculate values with QGIS model builder and field calculator

Question

I like to semiautomate my workflow.

I got always the same kind of geopackage with polygons and heat usages. I want to calculate the area of the polygons thus create a new field, calculate and write the area their and divide this by the heat usage in a second new cell.

I do not want a new file but create the new fields in the existing one.

I imagine it kind of this way

When I run it the following message pops up. "Could not creat layer building ... Creating of data source failed".

I found "attach to layer" but here I need to state where the geopackage layer is again. I like to speed up the process.

enter image description here

Is there another way?

enter image description here

• Bera: I dont think it is possible using Field Calculator. You can use pyqgis though
• Sethinacan: So your problem is overwriting the input layer? I have models that overwrite a layer in a geopackage with newly created layer - using an expression to select the correct gpkg and layer

Answer 1

As @Bera mentioned, this is difficult to do using only the Model Builder / Field Calculator.

With PyQGIS it is quite easy though:

# Get the active layer
layer = iface.activeLayer()
# Start editing the layer
layer.startEditing()
# Add new fields
layer.addAttribute(QgsField('area', QVariant.Double))
layer.addAttribute(QgsField('area_over_heat', QVariant.Double))
# Update fields
layer.updateFields()
# Get the index of the new fields and heat_usage (adapt name according to your field name in the following)
area_idx = layer.fields().indexOf('area')
div_idx = layer.fields().indexOf('area_over_heat')
field1_idx = layer.fields().indexOf('heat_usage') # add more fields for other calculations
# Calculate area & area_over_heat for each feature
for feature in layer.getFeatures():
 # Calculate area (in the layer's CRS units)
 area = feature.geometry().area()
 
 # Get field1 value
 field1_value = feature.attributes()[field1_idx]
 
 # Calculate division (handle potential division by zero)
 if field1_value != 0:
 div = area / field1_value
 else:
 div = None # or whatever value you want to use for division by zero
 
 # Update the feature
 layer.changeAttributeValue(feature.id(), area_idx, area)
 layer.changeAttributeValue(feature.id(), div_idx, div)
# Commit changes to your layer
layer.commitChanges()

• BAE_23: okay, I have no experience with pygis but I will try. thanks!