Conditions in modeler field calculator

Question

I want to check a vector file if there is a column "Source".

If yes, nothing to do. If there is no column "Source" it should be created and filled up with '0'.

Is this possible to do via the modeler?

Answer 1

1. Create a new script from:

   Processing Toolbox > Scripts > Tools > Create new script
   

2. Use something like the following:

   ##Example=name
   ##Layer=vector
   
   from qgis.core import QgsField
   from PyQt4.QtCore import QVariant
   
   layer = processing.getObject(Layer)
   idx = layer.fieldNameIndex('Source')
   
   if idx != -1:
       pass
   else:
       layer.startEditing()
       layer.dataProvider().addAttributes( [ QgsField("Source", QVariant.Int) ] )
       layer.updateFields()
       for feat in layer.getFeatures():
           layer.changeAttributeValue(feat.id(), layer.fieldNameIndex('Source'), '0')
       layer.commitChanges()
   

3. Save the script in your C:/Users/You/.qgis2/processing/scripts/ directory. Then you should be able to call this script in your modeler.