Default to display field when first setting labels enabled on a layer

src/app/qgslabelingwidget.cpp

@@ -186,6 +186,9 @@ void QgsLabelingWidget::labelModeChanged( int index )
     if ( !mSimpleSettings )
       mSimpleSettings.reset( new QgsPalLayerSettings() );
 
+    if ( mSimpleSettings->fieldName.isEmpty() )
+      mSimpleSettings->fieldName = mLayer->displayField();
+
     QgsLabelingGui *simpleWidget = new QgsLabelingGui( mLayer, mCanvas, *mSimpleSettings, this );
     simpleWidget->setDockMode( dockMode() );
     connect( simpleWidget, &QgsTextFormatWidget::widgetChanged, this, &QgsLabelingWidget::widgetChanged );

src/core/qgsvectorlayer.cpp

@@ -3106,7 +3106,7 @@ void QgsVectorLayer::setCoordinateSystem()
 
 QString QgsVectorLayer::displayField() const
 {
-  QgsExpression exp( mDisplayExpression );
+  QgsExpression exp( displayExpression() );
   if ( exp.isField() )
   {
     return static_cast<const QgsExpressionNodeColumnRef *>( exp.rootNode() )->name();
@@ -3134,30 +3134,34 @@ QString QgsVectorLayer::displayExpression() const
   {
     QString idxName;
 
-    const auto constMFields = mFields;
-    for ( const QgsField &field : constMFields )
+    // Check the fields and keep the first one that matches.
+    // We assume that the user has organized the data with the
+    // more "interesting" field names first. As such, name should
+    // be selected before oldname, othername, etc.
+    // This candidates list is a prioritized list of candidates ranked by "interestingness"!
+    // See discussion at https://github.com/qgis/QGIS/pull/30245 - this list must NOT be translated,
+    // but adding hardcoded localized variants of the strings is encouraged.
+    static QStringList sCandidates{ QStringLiteral( "name" ),
+                                    QStringLiteral( "heibt" ),
+                                    QStringLiteral( "desc" ),
+                                    QStringLiteral( "nom" ),
+                                    QStringLiteral( "street" ),
+                                    QStringLiteral( "road" ),
+                                    QStringLiteral( "id" )};
+    for ( const QString &candidate : sCandidates )
     {
-      QString fldName = field.name();
-
-      // Check the fields and keep the first one that matches.
-      // We assume that the user has organized the data with the
-      // more "interesting" field names first. As such, name should
-      // be selected before oldname, othername, etc.
-      if ( fldName.indexOf( QLatin1String( "name" ), 0, Qt::CaseInsensitive ) > -1 )
-      {
-        idxName = fldName;
-        break;
-      }
-      if ( fldName.indexOf( QLatin1String( "descrip" ), 0, Qt::CaseInsensitive ) > -1 )
+      for ( const QgsField &field : mFields )
       {
-        idxName = fldName;
-        break;
+        QString fldName = field.name();
+        if ( fldName.indexOf( candidate, 0, Qt::CaseInsensitive ) > -1 )
+        {
+          idxName = fldName;
+          break;
+        }
       }
-      if ( fldName.indexOf( QLatin1String( "id" ), 0, Qt::CaseInsensitive ) > -1 )
-      {
-        idxName = fldName;
+
+      if ( !idxName.isEmpty() )
         break;
-      }
     }
 
     if ( !idxName.isNull() )

tests/src/python/test_qgsvectorlayer.py

@@ -2949,6 +2949,26 @@ def testPrecisionAndDuplicateNodes(self):
         layer.addFeature(feature)
         self.assertGeometriesEqual(QgsGeometry.fromWkt('Polygon ((2596410 1224650, 2596400 1224650, 2596410 1224640, 2596410 1224650))'), feature.geometry(), 'geometry with unsnapped nodes', 'fixed geometry')
 
+    def testDefaultDisplayExpression(self):
+        """
+        Test that default display expression gravitates to most interesting column names
+        """
+        layer = QgsVectorLayer("Polygon?crs=epsg:2056&field=pk:int", "vl", "memory")
+        self.assertEqual(layer.displayExpression(), '"pk"')
+        self.assertEqual(layer.displayField(), 'pk')
+        layer = QgsVectorLayer("Polygon?crs=epsg:2056&field=pk:int&field=fid:int", "vl", "memory")
+        self.assertEqual(layer.displayExpression(), '"fid"')
+        self.assertEqual(layer.displayField(), 'fid')
+        layer = QgsVectorLayer("Polygon?crs=epsg:2056&field=pk:int&field=DESCRIPTION:string&field=fid:int", "vl", "memory")
+        self.assertEqual(layer.displayExpression(), '"DESCRIPTION"')
+        self.assertEqual(layer.displayField(), 'DESCRIPTION')
+        layer = QgsVectorLayer("Polygon?crs=epsg:2056&field=pk:int&field=DESCRIPTION:string&field=fid:int&field=NAME:string", "vl", "memory")
+        self.assertEqual(layer.displayExpression(), '"NAME"')
+        self.assertEqual(layer.displayField(), 'NAME')
+        layer = QgsVectorLayer("Polygon?crs=epsg:2056&field=pk:int&field=DESCRIPTION:string&field=fid:int&field=BETTER_NAME:string&field=NAME:string", "vl", "memory")
+        self.assertEqual(layer.displayExpression(), '"BETTER_NAME"')
+        self.assertEqual(layer.displayField(), 'BETTER_NAME')
+
 
 class TestQgsVectorLayerSourceAddedFeaturesInBuffer(unittest.TestCase, FeatureSourceTestCase):
 

tests/testdata/qgis_server/getprojectsettings.txt

@@ -344,7 +344,7 @@ Content-Type: text/xml; charset=utf-8
     <BoundingBox maxy="5.60603e+06" maxx="913215" miny="5.60601e+06" CRS="EPSG:3857" minx="913205"/>
     <BoundingBox maxy="8.20355" maxx="44.9015" miny="8.20346" CRS="EPSG:4326" minx="44.9014"/>
     <TreeName>groupwithoutshortname</TreeName>
-    <Layer geometryType="Point" queryable="0" displayField="id" visible="1">
+    <Layer geometryType="Point" queryable="0" displayField="name" visible="1">
      <Name>testlayer3</Name>
      <Title>testlayer3</Title>
      <CRS>CRS:84</CRS>