Fix aggregate expression calculation when used with virtual fields

The layer expression context (which is required for aggregate
calculation to work) was not being added to the context used
by vector layer feature iterators.

Fix #15930

python/core/qgsvectorlayerfeatureiterator.sip

@@ -61,6 +61,7 @@ class QgsVectorLayerFeatureSource : QgsAbstractFeatureSource
 
 
 
+
 };
 
 

src/core/qgsvectorlayerfeatureiterator.cpp

@@ -79,6 +79,9 @@ QgsVectorLayerFeatureSource::QgsVectorLayerFeatureSource( const QgsVectorLayer *
     }
 #endif
   }
+
+  std::unique_ptr< QgsExpressionContextScope > layerScope( QgsExpressionContextUtils::layerScope( layer ) );
+  mLayerScope = *layerScope;
 }
 
 QgsVectorLayerFeatureSource::~QgsVectorLayerFeatureSource()
@@ -644,7 +647,7 @@ void QgsVectorLayerFeatureIterator::prepareFields()
   mExpressionContext.reset( new QgsExpressionContext() );
   mExpressionContext->appendScope( QgsExpressionContextUtils::globalScope() );
   mExpressionContext->appendScope( QgsExpressionContextUtils::projectScope( QgsProject::instance() ) );
-  mExpressionContext->setFields( mSource->mFields );
+  mExpressionContext->appendScope( new QgsExpressionContextScope( mSource->mLayerScope ) );
 
   mFieldsToPrepare = ( mRequest.flags() & QgsFeatureRequest::SubsetOfAttributes ) ? mRequest.subsetOfAttributes() : mSource->mFields.allAttributesList();
 

src/core/qgsvectorlayerfeatureiterator.h

@@ -84,6 +84,8 @@ class CORE_EXPORT QgsVectorLayerFeatureSource : public QgsAbstractFeatureSource
 
     QgsFields mFields;
 
+    QgsExpressionContextScope mLayerScope;
+
     bool mHasEditBuffer;
 
     // A deep-copy is only performed, if the original maps change

tests/src/python/test_qgsvectorlayer.py

@@ -1829,6 +1829,27 @@ def testAggregate(self):
         self.assertTrue(ok)
         self.assertEqual(val, 'this is a test')
 
+    def testAggregateInVirtualField(self):
+        """
+        Test aggregates in a virtual field
+        """
+        layer = QgsVectorLayer("Point?field=fldint:integer", "layer", "memory")
+        pr = layer.dataProvider()
+
+        int_values = [4, 2, 3, 2, 5, None, 8]
+        features = []
+        for i in int_values:
+            f = QgsFeature()
+            f.setFields(layer.fields())
+            f.setAttributes([i])
+            features.append(f)
+        assert pr.addFeatures(features)
+
+        field = QgsField('virtual', QVariant.Double)
+        layer.addExpressionField('sum(fldint*2)', field)
+        vals = [f['virtual'] for f in layer.getFeatures()]
+        self.assertEqual(vals, [48, 48, 48, 48, 48, 48, 48])
+
     def onLayerOpacityChanged(self, tr):
         self.opacityTest = tr