Merge pull request #6312 from nyalldawson/field_exp_null

Don't silently discard requests to set QgsFieldExpressionWidget to an empty expression

Author: nyalldawson

src/gui/qgsfieldexpressionwidget.cpp

@@ -173,7 +173,10 @@ void QgsFieldExpressionWidget::setLayer( QgsMapLayer *layer )
 void QgsFieldExpressionWidget::setField( const QString &fieldName )
 {
   if ( fieldName.isEmpty() )
+  {
+    setRow( -1 );
     return;
+  }
 
   QModelIndex idx = mFieldProxyModel->sourceFieldModel()->indexFromName( fieldName );
   if ( !idx.isValid() )

tests/src/gui/testqgsfieldexpressionwidget.cpp

@@ -47,6 +47,7 @@ class TestQgsFieldExpressionWidget : public QObject
     void asExpression();
     void testIsValid();
     void testFilters();
+    void setNull();
 
   private:
     QgsFieldExpressionWidget *mWidget = nullptr;
@@ -269,6 +270,27 @@ void TestQgsFieldExpressionWidget::testFilters()
   QgsProject::instance()->removeMapLayer( layer );
 }
 
+void TestQgsFieldExpressionWidget::setNull()
+{
+  // test that QgsFieldExpressionWidget can be set to an empty value
+  QgsVectorLayer *layer = new QgsVectorLayer( QStringLiteral( "point?field=fld:int&field=fld2:int&field=fld3:int" ), QStringLiteral( "x" ), QStringLiteral( "memory" ) );
+  QgsProject::instance()->addMapLayer( layer );
+
+  std::unique_ptr< QgsFieldExpressionWidget > widget( new QgsFieldExpressionWidget() );
+  widget->setLayer( layer );
+
+  widget->setField( QString() );
+  QVERIFY( widget->currentField().isEmpty() );
+
+  widget->setField( QStringLiteral( "fld2" ) );
+  QCOMPARE( widget->currentField(), QStringLiteral( "fld2" ) );
+
+  widget->setField( QString() );
+  QVERIFY( widget->currentField().isEmpty() );
+
+  QgsProject::instance()->removeMapLayer( layer );
+}
+
 QGSTEST_MAIN( TestQgsFieldExpressionWidget )
 #include "testqgsfieldexpressionwidget.moc"