From 23ed6e9cd0e8c60b58bd26aaa78d531eb74550eb Mon Sep 17 00:00:00 2001
From: Nyall Dawson <nyall.dawson@gmail.com>
Date: Thu, 4 Feb 2021 17:04:18 +1000
Subject: [PATCH] Fix GeometryCollect aggregate detection of geometry result
 type when running from external python applications

---
 src/core/qgsaggregatecalculator.cpp | 36 ++++++++++++++++++-----------
 1 file changed, 22 insertions(+), 14 deletions(-)

diff --git a/src/core/qgsaggregatecalculator.cpp b/src/core/qgsaggregatecalculator.cpp
index 998e0dff261..f1c74e3b708 100644
--- a/src/core/qgsaggregatecalculator.cpp
+++ b/src/core/qgsaggregatecalculator.cpp
@@ -102,23 +102,31 @@ QVariant QgsAggregateCalculator::calculate( QgsAggregateCalculator::Aggregate ag
   QVariant::Type resultType = QVariant::Double;
   if ( attrNum == -1 )
   {
-    // evaluate first feature, check result type
-    QgsFeatureRequest testRequest( request );
-    testRequest.setLimit( 1 );
-    QgsFeature f;
-    QgsFeatureIterator fit = mLayer->getFeatures( testRequest );
-    if ( !fit.nextFeature( f ) )
+    if ( aggregate == GeometryCollect )
     {
-      //no matching features
-      if ( ok )
-        *ok = true;
-      return defaultValue( aggregate );
+      // in this case we know the result should be a geometry value, so no need to sniff it out...
+      resultType = QVariant::UserType;
     }
+    else
+    {
+      // evaluate first feature, check result type
+      QgsFeatureRequest testRequest( request );
+      testRequest.setLimit( 1 );
+      QgsFeature f;
+      QgsFeatureIterator fit = mLayer->getFeatures( testRequest );
+      if ( !fit.nextFeature( f ) )
+      {
+        //no matching features
+        if ( ok )
+          *ok = true;
+        return defaultValue( aggregate );
+      }
 
-    if ( context )
-      context->setFeature( f );
-    QVariant v = expression->evaluate( context );
-    resultType = v.type();
+      if ( context )
+        context->setFeature( f );
+      QVariant v = expression->evaluate( context );
+      resultType = v.type();
+    }
   }
   else
     resultType = mLayer->fields().at( attrNum ).type();