[feature][processing] Add a last_value function to the aggregate algo…

…rithm

src/analysis/processing/qgsalgorithmaggregate.cpp

@@ -83,7 +83,7 @@ bool QgsAggregateAlgorithm::prepareAlgorithm( const QVariantMap &parameters, Qgs
   mGeometryExpression = createExpression( QStringLiteral( "collect($geometry, %1)" ).arg( mGroupBy ), context );
 
   const QVariantList aggregates = parameters.value( QStringLiteral( "AGGREGATES" ) ).toList();
-
+  int currentAttributeIndex = 0;
   for ( const QVariant &aggregate : aggregates )
   {
     const QVariantMap aggregateDef = aggregate.toMap();
@@ -109,6 +109,11 @@ bool QgsAggregateAlgorithm::prepareAlgorithm( const QVariantMap &parameters, Qgs
     {
       expression = source;
     }
+    else if ( aggregateType == QLatin1String( "last_value" ) )
+    {
+      expression = source;
+      mAttributesRequireLastFeature << currentAttributeIndex;
+    }
     else if ( aggregateType == QLatin1String( "concatenate" ) || aggregateType == QLatin1String( "concatenate_unique" ) )
     {
       expression = QStringLiteral( "%1(%2, %3, %4, \'%5\')" ).arg( aggregateType,
@@ -122,6 +127,7 @@ bool QgsAggregateAlgorithm::prepareAlgorithm( const QVariantMap &parameters, Qgs
       expression = QStringLiteral( "%1(%2, %3)" ).arg( aggregateType, source, mGroupBy );
     }
     mExpressions.append( createExpression( expression, context ) );
+    currentAttributeIndex++;
   }
 
   return true;
@@ -157,8 +163,8 @@ QVariantMap QgsAggregateAlgorithm::processAlgorithm( const QVariantMap &paramete
     // upgrade group by value to a list, so that we get correct behavior with the QHash
     const QVariantList key = groupByValue.type() == QVariant::List ? groupByValue.toList() : ( QVariantList() << groupByValue );
 
-    auto groupIt = groups.constFind( key );
-    if ( groupIt == groups.constEnd() )
+    auto groupIt = groups.find( key );
+    if ( groupIt == groups.end() )
     {
       QString id = QStringLiteral( "memory:" );
       std::unique_ptr< QgsFeatureSink > sink( QgsProcessingUtils::createFeatureSink( id,
@@ -176,13 +182,14 @@ QVariantMap QgsAggregateAlgorithm::processAlgorithm( const QVariantMap &paramete
       //store ownership of sink in groupSinks, so that these get deleted automatically if an exception is raised later..
       groupSinks.emplace_back( std::move( sink ) );
       group.layer = layer;
-      group.feature = feature;
+      group.firstFeature = feature;
       groups[key] = group;
       keys.append( key );
     }
     else
     {
       groupIt->sink->addFeature( feature, QgsFeatureSink::FastInsert );
+      groupIt->lastFeature = feature;
     }
 
     current++;
@@ -210,7 +217,7 @@ QVariantMap QgsAggregateAlgorithm::processAlgorithm( const QVariantMap &paramete
 
     QgsExpressionContext exprContext = createExpressionContext( parameters, context );
     exprContext.appendScope( QgsExpressionContextUtils::layerScope( group.layer ) );
-    exprContext.setFeature( group.feature );
+    exprContext.setFeature( group.firstFeature );
 
     QgsGeometry geometry = mGeometryExpression.evaluate( &exprContext ).value< QgsGeometry >();
     if ( mGeometryExpression.hasEvalError() )
@@ -234,8 +241,10 @@ QVariantMap QgsAggregateAlgorithm::processAlgorithm( const QVariantMap &paramete
 
     QgsAttributes attributes;
     attributes.reserve( mExpressions.size() );
+    int currentAttributeIndex = 0;
     for ( auto it = mExpressions.begin(); it != mExpressions.end(); ++it )
     {
+      exprContext.setFeature( mAttributesRequireLastFeature.contains( currentAttributeIndex ) ? group.lastFeature : group.firstFeature );
       if ( it->isValid() )
       {
         const QVariant value = it->evaluate( &exprContext );
@@ -249,6 +258,7 @@ QVariantMap QgsAggregateAlgorithm::processAlgorithm( const QVariantMap &paramete
       {
         attributes.append( QVariant() );
       }
+      currentAttributeIndex++;
     }
 
     // Write output feature

src/analysis/processing/qgsalgorithmaggregate.h

@@ -62,13 +62,15 @@ class QgsAggregateAlgorithm : public QgsProcessingAlgorithm
 
     QgsFields mFields;
     QList< QgsExpression > mExpressions;
+    QList< int > mAttributesRequireLastFeature;
     QgsDistanceArea mDa;
 
     struct Group
     {
       QgsFeatureSink *sink = nullptr;
       QgsMapLayer *layer = nullptr;
-      QgsFeature feature;
+      QgsFeature firstFeature;
+      QgsFeature lastFeature;
     };
 
 };

src/gui/processing/qgsprocessingaggregatewidgets.cpp

@@ -566,7 +566,8 @@ const QStringList QgsAggregateMappingWidget::AggregateDelegate::aggregates()
   static std::once_flag initialized;
   std::call_once( initialized, [ = ]( )
   {
-    sAggregates << QStringLiteral( "first_value" );
+    sAggregates << QStringLiteral( "first_value" )
+                << QStringLiteral( "last_value" );
 
     const QList<QgsExpressionFunction *> functions = QgsExpression::Functions();
     for ( const QgsExpressionFunction *function : functions )