Iterate over features in a model

Question

In QGIS Modeler I want to iterate over vector feature in two layers so that each iteration the inputs are:

1. Layer 1 feature 1 - Layer 2 feature 1
2. Layer 1 feature 2 - Layer 2 feature 2
3. Layer 1 feature 3 - Layer 2 feature 3 and so on

My attempt: I have a simple model which has two Vector Features Inputs and activate "Iterate over this Layer..:" on both inputs.

The result is:

1. Layer 1 feature 1 - Layer 2 all features
2. Layer 1 feature 2 - Layer 2 all features ...

Is this possible?

Answer 1

I don't know if it is possible using actual options in Modeler or Processing window, but you can make a script doing that.

from qgis.PyQt.QtCore import QCoreApplication
from qgis.core import (QgsProcessing,
                       QgsFeatureSink,
                       QgsProcessingAlgorithm,
                       QgsProcessingParameterFeatureSource,
                       QgsProcessingParameterFeatureSink)


class ExampleProcessingAlgorithm(QgsProcessingAlgorithm):
    INPUT1 = 'INPUT1'
    INPUT2 = 'INPUT2'
    OUTPUT = 'OUTPUT'

    def tr(self, string):
        return QCoreApplication.translate('Processing', string)

    def createInstance(self):
        return ExampleProcessingAlgorithm()

    def name(self):
        return 'myscript'

    def displayName(self):
        return self.tr('Custom Difference')

    def group(self):
        return self.tr('Example scripts')

    def groupId(self):
        return 'examplescripts'

    def shortHelpString(self):
        return self.tr("Example algorithm short description")

    def initAlgorithm(self, config=None):
        self.addParameter(QgsProcessingParameterFeatureSource(self.INPUT1, self.tr('Layer1')))
        self.addParameter(QgsProcessingParameterFeatureSource(self.INPUT2, self.tr('Layer2')))
        self.addParameter(QgsProcessingParameterFeatureSink(self.OUTPUT, self.tr('Output')))

    def processAlgorithm(self, parameters, context, feedback):
        layer1 = self.parameterAsVectorLayer(parameters, self.INPUT1, context)
        layer2 = self.parameterAsVectorLayer(parameters, self.INPUT2, context)
        
        (sink, dest_id) = self.parameterAsSink(parameters, self.OUTPUT, context,
                                               layer1.fields(), layer1.wkbType(), layer1.sourceCrs())
  
        for i in range(layer1.featureCount()):
            feat1 = layer1.getFeature(i)
            feat2 = layer2.getFeature(i)  
            
            ### main processsing section ###
            geom = feat1.geometry().difference(feat2.geometry())
            feat1.setGeometry(geom)
            ################################

            sink.addFeature(feat1, QgsFeatureSink.FastInsert)

        return {self.OUTPUT: dest_id}

Then use it in QGIS Modeler:

Inputs: (Upper polygons in two layers have the same FID value. Same as lower polygons)

Output: