Use id in gui if name is empty and crop layerid-part in the generated id

The name can be empty, so use the id instead in the combo box. To avoid so long ids, the part taken from the layerid in the relation id is cropped to 10 signs. They are unique anyway because there is used a suffix counting up.

src/app/qgsrelationmanagerdialog.cpp

@@ -92,9 +92,9 @@ void QgsRelationManagerDialog::on_mBtnAddRelation_clicked()
     QString relationId = addDlg.relationId();
     if ( addDlg.relationId() == "" )
       relationId = QString( "%1_%2_%3_%4" )
-                   .arg( addDlg.referencingLayerId(),
+                   .arg( addDlg.referencingLayerId().left( 10 ),
                          addDlg.references().at( 0 ).first,
-                         addDlg.referencedLayerId(),
+                         addDlg.referencedLayerId().left( 10 ),
                          addDlg.references().at( 0 ).second );
 
     QStringList existingNames;

src/gui/editorwidgets/qgsrelationreferenceconfigdlg.cpp

@@ -47,7 +47,11 @@ QgsRelationReferenceConfigDlg::QgsRelationReferenceConfigDlg( QgsVectorLayer* vl
 
   Q_FOREACH ( const QgsRelation& relation, vl->referencingRelations( fieldIdx ) )
   {
-    mComboRelation->addItem( QString( "%1 (%2)" ).arg( relation.name(), relation.referencedLayerId() ), relation.id() );
+    if ( relation.name().isEmpty() )
+      mComboRelation->addItem( QString( "%1 (%2)" ).arg( relation.id(), relation.referencedLayerId() ), relation.id() );
+    else
+      mComboRelation->addItem( QString( "%1 (%2)" ).arg( relation.name(), relation.referencedLayerId() ), relation.id() );
+
     if ( relation.referencedLayer() )
     {
       mExpressionWidget->setField( relation.referencedLayer()->displayExpression() );