Fix for zonal statistics (ticket #4972)

src/analysis/vector/qgszonalstatistics.cpp

@@ -109,9 +109,9 @@ int QgsZonalStatistics::calculateStatistics( QProgressDialog* p )
 
   //add the new count, sum, mean fields to the provider
   QList<QgsField> newFieldList;
-  QgsField countField( mAttributePrefix + "count", QVariant::Double );
-  QgsField sumField( mAttributePrefix + "sum", QVariant::Double );
-  QgsField meanField( mAttributePrefix + "mean", QVariant::Double );
+  QgsField countField( mAttributePrefix + "count", QVariant::Double, "double precision" );
+  QgsField sumField( mAttributePrefix + "sum", QVariant::Double, "double precision" );
+  QgsField meanField( mAttributePrefix + "mean", QVariant::Double, "double precision" );
   newFieldList.push_back( countField );
   newFieldList.push_back( sumField );
   newFieldList.push_back( meanField );

src/plugins/zonal_statistics/qgszonalstatisticsdialog.cpp

@@ -68,7 +68,11 @@ void QgsZonalStatisticsDialog::insertAvailableLayers()
       QgsVectorLayer* vl = dynamic_cast<QgsVectorLayer*>( layer_it.value() );
       if ( vl && vl->geometryType() == QGis::Polygon )
       {
-        mPolygonLayerComboBox->addItem( vl->name(), QVariant( vl->id() ) );
+        QgsVectorDataProvider* provider  = vl->dataProvider();
+        if ( provider->capabilities() & QgsVectorDataProvider::AddAttributes )
+        {
+          mPolygonLayerComboBox->addItem( vl->name(), QVariant( vl->id() ) );
+        }
       }
     }
   }