prevent parts from being dragged into parts

src/Gui/ViewProviderPart.cpp

@@ -72,6 +72,16 @@ bool ViewProviderPart::doubleClicked(void)
     return true;
 }
 
+bool ViewProviderPart::canDropObject(App::DocumentObject* obj) const {
+    
+    //it is not allowed to have any part or assembly object within a part, hence we exclude origin groups
+    if(obj->isDerivedFrom(App::OriginGroup::getClassTypeId()))
+        return false;
+    
+    return Gui::ViewProvider::canDropObject(obj);
+}
+
+
 /**
  * Returns the pixmap for the list item.
  */