attribute form: lookup attribute fields by name. joined layers might be unavailable at load time (fixes #12146)

Author: jef-n

src/core/qgsvectorlayer.cpp

@@ -1758,7 +1758,7 @@ QgsAttributeEditorElement* QgsVectorLayer::attributeEditorElementFromDomElement(
     for ( int i = 0; i < childNodeList.size(); i++ )
     {
       QDomElement childElem = childNodeList.at( i ).toElement();
-      QgsAttributeEditorElement* myElem = attributeEditorElementFromDomElement( childElem, container );
+      QgsAttributeEditorElement *myElem = attributeEditorElementFromDomElement( childElem, container );
       if ( myElem )
         container->addChildElement( myElem );
     }
@@ -1768,7 +1768,7 @@ QgsAttributeEditorElement* QgsVectorLayer::attributeEditorElementFromDomElement(
   else if ( elem.tagName() == "attributeEditorField" )
   {
     QString name = elem.attribute( "name" );
-    int idx = *( dataProvider()->fieldNameMap() ).find( name );
+    int idx = fieldNameIndex( name );
     newElement = new QgsAttributeEditorField( name, idx, parent );
   }
   else if ( elem.tagName() == "attributeEditorRelation" )

src/gui/qgsattributeform.cpp

@@ -571,7 +571,7 @@ QWidget* QgsAttributeForm::createWidgetFromDef( const QgsAttributeEditorElement
       if ( !fieldDef )
         break;
 
-      int fldIdx = fieldDef->idx();
+      int fldIdx = vl->fieldNameIndex( fieldDef->name() );
       if ( fldIdx < vl->pendingFields().count() && fldIdx >= 0 )
       {
         const QString widgetType = mLayer->editorWidgetV2( fldIdx );