Avoid freezing attribute form with recursion

The search widget wrappers for relations have two issues

* They recursively load whatever relations are defined. With self-referencing this leads to 💀
  This is addressed by only loading one level of relations in search widgets.

* They would load even when hidden, leading to long load times on attribute table opening.
  We now only actually load the form on the show event

python/gui/editorwidgets/qgsrelationaggregatesearchwidgetwrapper.sip.in

@@ -42,6 +42,7 @@ Constructor
 
     virtual void setExpression( const QString &value );
 
+    virtual bool eventFilter( QObject *watched, QEvent *event );
 
 };
 

src/gui/editorwidgets/qgsrelationaggregatesearchwidgetwrapper.cpp

@@ -61,10 +61,9 @@ QWidget *QgsRelationAggregateSearchWidgetWrapper::createWidget( QWidget *parent
   }
   else
   {
-    QgsAttributeEditorContext subContext = QgsAttributeEditorContext( context(), mWrapper->relation(), QgsAttributeEditorContext::Multiple, QgsAttributeEditorContext::Embed );
-    mAttributeForm = new QgsAttributeForm( mWrapper->relation().referencingLayer(), QgsFeature(), subContext, parent );
-    mAttributeForm->setMode( QgsAttributeForm::AggregateSearchMode );
-    widget = mAttributeForm;
+    mContainerWidget = new QWidget( parent );
+    widget = mContainerWidget;
+    widget->installEventFilter( this );
   }
 
   groupBox->setLayout( new QGridLayout() );
@@ -83,3 +82,19 @@ void QgsRelationAggregateSearchWidgetWrapper::setExpression( const QString &valu
   Q_UNUSED( value )
   QgsDebugMsg( "Not supported" );
 }
+
+bool QgsRelationAggregateSearchWidgetWrapper::eventFilter( QObject *watched, QEvent *event )
+{
+  bool rv = QgsSearchWidgetWrapper::eventFilter( watched, event );
+  if ( event->type() == QEvent::Show && !mAttributeForm )
+  {
+    QgsAttributeEditorContext subContext = QgsAttributeEditorContext( context(), mWrapper->relation(), QgsAttributeEditorContext::Multiple, QgsAttributeEditorContext::Embed );
+    mAttributeForm = new QgsAttributeForm( mWrapper->relation().referencingLayer(), QgsFeature(), subContext, mContainerWidget );
+    mAttributeForm->setMode( QgsAttributeForm::AggregateSearchMode );
+    QGridLayout *glayout = new QGridLayout();
+    mContainerWidget->setLayout( glayout );
+    glayout->setMargin( 0 );
+    glayout->addWidget( mAttributeForm );
+  }
+  return rv;
+}

src/gui/editorwidgets/qgsrelationaggregatesearchwidgetwrapper.h

@@ -49,10 +49,12 @@ class GUI_EXPORT QgsRelationAggregateSearchWidgetWrapper : public QgsSearchWidge
     QWidget *createWidget( QWidget *parent ) override;
     bool applyDirectly() override;
     void setExpression( const QString &value ) override;
+    virtual bool eventFilter( QObject *watched, QEvent *event ) override;
 
   private:
     QgsRelationWidgetWrapper *mWrapper = nullptr;
     QgsAttributeForm *mAttributeForm = nullptr;
+    QWidget *mContainerWidget = nullptr;
 };
 
 #endif // QGSRELATIONAGGREGATESEARCHWIDGETWRAPPER_H

src/gui/qgsattributeformrelationeditorwidget.cpp

@@ -28,10 +28,13 @@ QgsAttributeFormRelationEditorWidget::QgsAttributeFormRelationEditorWidget( QgsR
 
 void QgsAttributeFormRelationEditorWidget::createSearchWidgetWrappers( const QgsAttributeEditorContext &context )
 {
-  Q_UNUSED( context )
-  mSearchWidget = new QgsRelationAggregateSearchWidgetWrapper( layer(), mWrapper, form() );
+  if ( context.parentContext() )
+  {
+    mSearchWidget = new QgsRelationAggregateSearchWidgetWrapper( layer(), mWrapper, form() );
+    mSearchWidget->setContext( context );
 
-  setSearchWidgetWrapper( mSearchWidget );
+    setSearchWidgetWrapper( mSearchWidget );
+  }
 }
 
 QString QgsAttributeFormRelationEditorWidget::currentFilterExpression() const